Zinc hot dip galvanized steel plate excellent in press formability and method for production thereof

ABSTRACT

A hot-dip galvanized steel sheet includes a plating layer substantially composed of the η phase and an oxide layer disposed on a surface of the plating layer. The oxide layer has an average thickness of 10 nm or more and includes a Zn-based oxide layer and an Al-based oxide layer. A method for producing the hot-dip galvanized steel sheet includes a hot-dip galvanization step, a temper rolling step, and an oxidation step.

This application is the United States national phase application ofInternational Application PCT/JP2003/013281 filed Oct. 17, 2003.

FIELD OF THE INVENTION

The present invention relates to hot-dip galvanized steel sheets havingexcellent press formability and methods for producing the same.

DESCRIPTION OF THE RELATED ARTS

Recently, in view of improvement in rust preventive properties, the rateof use of zinc-based plated steel sheets, in particular, hot-dipzinc-based coated steel sheets, for automotive panels has beenincreasing. Hot-dip zinc-based coated steel sheets are classified intothose subjected to alloying treatment after being galvanized and thosenot subjected to alloying treatment. In general, the former are referredto as hot-dip galvannealed steel sheets and the latter are referred toas hot-dip galvanized steel sheets. Usually, as the hot-dip zinc-basedcoated steel sheets for automotive panels, hot-dip galvannealed steelsheets which are produced by hot-dip galvanizing and subsequent alloyingtreatment at about 500° C. are usually used because of their excellentweldability and paintability.

In order to further improve rust-preventive properties, there has beenan increased demand from automotive manufacturers for zinc-based platedsteel sheets with a heavy coating weight. If the coating weight of thehot-dip galvannealed steel sheets is increased, a long time is requiredfor alloying, and incomplete alloying, i.e., so-called uneven burning,easily occurs. On the other hand, if alloying is attempted to becompleted over the entire plating layer, overalloying occurs. As aresult, a brittle Γ phase is generated at the interface between theplating layer and the steel sheet, and plating peeling is likely tooccur during working. Therefore, it is extremely difficult to producehot-dip galvannealed steel sheets with a heavy coating weight.

Consequently, hot-dip galvanized steel sheets are effective in allowingthe coating weight to be increased. However, when a hot-dip galvanizedsteel sheet is press-formed into an automotive panel, sliding frictionwith a die is larger compared with a hot-dip galvannealed steel sheet.Since the melting point of the surface is low, adhesion is likely tooccur, resulting in cracking during pressing.

In order to solve such problems, Japanese Unexamined Patent PublicationNo. 2002-4019 (Patent Literature 1) and Japanese Unexamined PatentPublication No. 2002-4020 (Patent Literature 2) disclose a technique inwhich die galling is prevented at the time of press forming bycontrolling the surface roughness of the hot-dip galvanized steel sheetand a technique in which deep drawability is improved. As a result ofextensive research of such hot-dip galvanized steel sheets, it has beenfound that when a hot-dip galvanized steel sheet slides over a die andwhen the sliding distance is short, adhesion to the die is prevented.However, as the sliding distance is increased, such an effect isweakened, and depending on the sliding conditions, no improvement effectis achieved. In the disclosures described above, in order to impartroughness to the hot-dip galvanized steel sheet, a method is describedin which roller conditions and rolling conditions in skin-pass rollingare controlled. In practice, since rollers become clogged with zinc, itis difficult to impart a predetermined roughness to the surface of thehot-dip galvanized steel sheet stably.

Japanese Unexamined Patent Publication No. 2-190483 (Patent Literature3) discloses a galvanized steel sheet in which an oxide layer primarilycomposed of ZnO is formed on the surface of the plating layer. However,it is difficult to apply this technique to a hot-dip galvanized steelsheet. When a hot-dip galvanized steel sheet is produced, usually, avery small amount of Al is incorporated into a zinc bath so as toprevent an excessive Fe—Zn alloying reaction and to secure platingadhesion during dipping in the zinc bath. Because of the very smallamount of Al involved, an Al-based oxide layer is densely generated onthe surface of the hot-dip galvanized steel sheet. Therefore, thesurface is inactive and it is not possible to form an oxide layerprimarily composed of ZnO on the surface. Even if such an oxide layer isapplied onto the densely generated Al-based oxide layer, adhesionbetween the applied oxide layer and the substrate is poor, and thus itis not possible to achieve a satisfactory effect. The oxide layer isalso likely to adhere to the press die during working, resulting inadverse effects on the pressed article, for example, the formation ofdents.

In addition, Japanese Unexamined Patent Publication No. 3-191091 (PatentLiterature 4) discloses a galvanized steel sheet provided with an Mooxide layer on the surface, Japanese Unexamined Patent Publication No.3-191092 (Patent Literature 5) discloses a galvanized steel sheetprovided with a Co oxide layer on the surface, Japanese UnexaminedPatent Publication No. 3-191093 (Patent Literature 6) discloses agalvanized steel sheet provided with a Ni oxide layer on the surface,and Japanese Unexamined Patent Publication No. 3-191094 (PatentLiterature 7) discloses a galvanized steel sheet provided with a Caoxide layer on the surface. However, for the same reason as for theoxide layer primarily composed of ZnO, it is not possible to achieve asatisfactory effect.

Japanese Unexamined Patent Publication No. 2000-160358 (PatentLiterature 8) discloses a galvanized steel sheet provided with an oxidelayer composed of an Fe oxide, a Zn oxide, and an Al oxide. As in thecase described above, with respect to the hot-dip galvanized steelsheet, since the surface is inactive, the Fe oxide initially formedbecomes nonuniform. A large amount of oxides is also required to achievea satisfactory effect, resulting in peeling of the oxides.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a hot-dip galvanizedsteel sheet in which the sliding friction is small during press formingand which exhibits stable, excellent press formability and a method forproducing the same.

In order to achieve the object, the present invention provides a hot-dipgalvanized steel sheet, comprising a plating layer consistingessentially a η phase and an oxide layer disposed on a surface of theplating layer, the oxide layer having an average thickness of 10 nm ormore. Preferably, the oxide layer has an average thickness of 10 to 200nm. The oxide layer includes a Zn-based oxide layer having a Zn/Alatomic concentration ratio of more than 1 and an Al-based oxide layerhaving a Zn/Al atomic concentration ratio of less than 1.

It is preferable that the plating layer has concavities and convexitieson the surface, and the Zn-based oxide layer is disposed at least on theconcavities.

It is preferable that the Zn-based oxide layer has microirregularities,which has a mean spacing (S) determined based on a roughness curve of1,000 nm or less and an average roughness (Ra) of 100 nm or less.

Preferably, the Zn-based oxide layer has microirregularities with anetwork structure including convexities and discontinuous concavitiessurrounded by the convexities.

Preferably, the Zn-based oxide layer includes an oxide containing Zn andFe and the Fe concentration defined by the expression Fe/(Zn+Fe) is 1 to50 atomic percent.

Preferably, the Zn-based oxide layer has an areal rate of 15% or morewith respect to the surface of the plating layer.

In the hot-dip galvanized steel sheet of the present invention,preferably, the Zn-based oxide layer has a Zn/Al atomic concentrationratio of 4 or more. In the case when the Zn/Al ratio is 4 or more, morepreferably, the following conditions are satisfied.

(A) The Zn-based oxide layer has an areal rate of 70% or more withrespect to the surface of the plating layer.

(B) The Zn-based oxide layer is disposed on the concavities of thesurface of the plating layer formed by temper rolling, and on theconvexities or planar portions other than the convexities.

(C) The Zn-based oxide layer includes an oxide containing Zn and Fe andthe Fe concentration ratio defined by the expression Fe/(Zn+Fe) is 1 to50 atomic percent.

(D) The Zn-based oxide layer has microirregularities with a networkstructure including convexities and discontinuous concavities surroundedby the convexities.

Also, the present invention provides a hot-dip galvanized steel sheetincluding a plating layer consisting essentially of a η phase and aZn-based oxide layer containing Fe disposed on a surface of the platinglayer, the Zn-based oxide layer having an Fe atomic ratio of 1 to 50atomic percent, the Fe atomic ratio being defined as Fe/(Fe+Zn).

Preferably, the Zn-based oxide layer has microirregularities with anetwork structure including convexities and discontinuous concavitiessurrounded by the convexities.

Preferably, the Zn-based oxide layer has an areal rate of 15% or morewith respect to the surface of the plating layer.

Moreover, the present invention provides a hot-dip galvanized steelsheet including a plating layer consisting essentially of a η phase anda Zn-based oxide layer containing Fe disposed on a surface of theplating layer, the Zn-based oxide layer having microirregularities witha network structure including convexities and discontinuous concavitiessurrounded by the convexities.

Preferably, the Zn-based oxide layer has a mean spacing (S) determinedbased on a roughness curve of 10 to 1,000 nm and an average roughness(Ra) of 4 to 100 nm.

Preferably, the Zn-based oxide layer has an areal rate of 70% or morewith respect to the surface of the plating layer.

Preferably, the Zn-based oxide layer is disposed on the planar portionsof the surface of the plating layer other than the concavities formed bytemper rolling. More preferably, in the Zn-based oxide layer disposed onthe planar portions, the mean spacing (S) determined based on theroughness curve is 10 to 500 nm and the average roughness (Ra)determined based on the roughness curve is 4 to 100 nm.

Additionally, in the present invention, the “Zn-based oxide” present onthe surface of the plating layer may include a Zn-based oxide only, mayalso include a Zn-based hydroxide, or may include a Zn-based hydroxideonly.

Further, the present invention provides a method for producing a hot-dipgalvanized steel sheet including a hot-dip galvanization step, a temperrolling step, and an oxidation treatment step. In the hot-dipgalvanization step, a steel sheet is hot-dip galvanized to form ahot-dip galvanized layer. In the temper rolling step, the steel sheetprovided with the hot-dip galvanized layer is temper-rolled. In theoxidation treatment step, the temper-rolled steel sheet is brought intocontact with an acidic solution having a pH buffering effect andretained for 1 to 30 seconds before washing with water to performoxidation treatment. Preferably, the acidic solution contains 1 to 200g/l of Fe ions.

Preferably, the method for producing the hot-dip galvanized steel sheetfurther includes an activation step for activating the surface before orafter the temper rolling step. More preferably, the activation step isperformed before the temper rolling step. Preferably, the activationstep includes bringing the steel sheet into contact with an alkalinesolution with a pH of 11 or more at 50° C. or more for 1 second or more.By the activation step, the Al-based oxide content in a surface oxidelayer before the oxidation treatment step is controlled so that the Alconcentration is less than 20 atomic percent.

Also, the present invention provides a method for producing a hot-dipgalvanized steel sheet including a hot-dip galvanization step ofhot-dip-galvanizing a steel sheet to form a hot-dip galvanized layer; atemper rolling step of temper-rolling the steel sheet provided with thehot-dip galvanized layer; an oxidation treatment step of oxidizing thetemper-rolled steel sheet by bringing the temper-rolled steel sheet intocontact with an acidic solution having a pH buffering effect andcontaining 5 to 200 g/l of Fe ions with a pH of 1 to 3, and retainingthe temper-rolled steel sheet in this solution for 1 to 30 secondsbefore washing with water; and an activation step of activating thesurface before or after the temper rolling step.

In another aspect of the present invention, a method for producing ahot-dip galvanized steel sheet includes a hot-dip galvanization step ofhot-dip-galvanizing a steel sheet to form a hot-dip galvanized layer; atemper rolling step of temper-rolling the steel sheet provided with thehot-dip galvanized layer; an oxidation treatment step of oxidizing thetemper-rolled steel sheet by bringing the temper-rolled steel sheet intocontact with an acidic solution having a pH buffering effect with a pHof 1 to 5, and retaining the temper-rolled steel sheet in this solutionfor 1 to 30 seconds before washing with water; and an activation step ofactivating the surface before or after the temper rolling step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation view which schematically shows a frictioncoefficient measuring device.

FIG. 2 is a perspective view which schematically shows the shape anddimension of a bead shown in FIG. 1.

FIG. 3 is a graph which shows an Auger profile of the surface of SampleNo. 1 shown in Table 4 in Embodiment 2 after activation and beforeoxidation.

FIG. 4 is a graph which shows an Auger profile of the surface of SampleNo. 11 shown in Table 4 in Embodiment 2 after activation and beforeoxidation.

FIG. 5 is a graph which shows an Auger profile of the surface of SampleNo. 12 shown in Table 4 in Embodiment 2 after activation and beforeoxidation.

EMBODIMENT FOR CARRYING OUT THE INVENTION Embodiment 1

The present inventors have found that it is possible to obtainsatisfactory press formability under extended sliding conditions byforming a Zn-based oxide along with an inherent Al-based oxide on thesurface of a hot-dip galvanized steel sheet.

As described above, since an Al-based oxide layer is formed on thesurface of a hot-dip galvanized steel sheet, it is possible to preventadhesion between the steel sheet and a die during press forming.Therefore, it is believed to be effective in forming a thicker Al-basedoxide layer in order to further improve sliding performance during pressforming. However, in order to form a thick Al-based oxide layer, thesteel sheet must be oxidized at high temperatures for a long period oftime, which is practically difficult. During such an oxidation period,an Fe—Zn alloying reaction advances gradually, resulting in degradationin plating adhesion. On the other hand, in order to form a Zn-basedoxide layer, the Al-based oxide layer on the surface must be removedcompletely, and it takes a long time to perform such treatment.

If the Al-based oxide layer is partially broken down to expose a newsurface and surface oxidation treatment is performed, a Zn-based oxideis formed on the newly exposed surface, and it is also possible to applya Zn-based oxide layer to the newly exposed surface. In the oxide layerthus formed on the surface of the plating layer, both the Zn-based oxideand the Al-based oxide are present, and thereby adhesion to the pressdie is further prevented. Consequently, it is possible to obtainsatisfactory press formability under the extended sliding conditions. Ithas also been found that by forming such a Zn-based oxide layer at leaston the concavities in the irregularities formed on the surface of theplating layer, sliding friction can be reduced.

In the oxidation treatment, by immersing the hot-dip galvanized steelsheet in an acidic solution so as to form an acidic solution film on thesurface of the steel sheet and then by allowing it to stand for apredetermined time, it is possible to form the Zn-based oxideeffectively. Additionally, after temper rolling is performed, bybringing the steel sheet into contact with an alkaline solution so as topartially break down and dissolve the Al-based oxide layer, the oxidelayer can be more effectively formed.

The present inventors have also found that by formingmicroirregularities in the Zn-based oxide disposed on the surface of theplating layer, sliding performance can be further improved. Themicroirregularities are defined by a surface roughness in which theaverage roughness Ra (hereinafter also referred to simply as “Ra”)determined based on the roughness curve is 100 nm or less and the meanspacing S (hereinafter also referred to simply as “S”) of localirregularities determined based on the roughness curve is 1,000 nm orless. This surface roughness is one or more orders of magnitude smallerthan the surface roughness (Ra: about 1 μm) described in the PatentLiterature 1 or 2. Accordingly, the surface roughness parameters, suchas Ra, in the present invention are calculated based on the roughnesscurve with a length of several microns, and are different from thegeneral surface roughness parameters which define irregularities of themicron (μm) order or more determined based on the roughness curve with alength of the millimeter order or more. In the related literatures, thesurface roughness of the hot-dip galvanized steel sheet is defined,while in the present invention, the surface roughness of the oxide layerapplied to the surface of the hot-dip galvanized steel sheet is defined.

The present inventors have also found that in order to formmicroirregularities in the Zn-based oxide, it is effective toincorporate Fe into the Zn-based oxide. In the method in which theacidic solution film is formed on the surface of the steel sheet andthen the steel sheet is allowed to stand for a predetermined time sothat the Zn-based oxide is added to the hot-dip galvanized steel sheet,by incorporating Fe into the acidic solution, the Zn-based oxidecontaining Zn and Fe is formed, and thereby microirregularities can beeffectively formed in the oxide.

Since the hot-dip galvanized steel sheet is usually produced by dippinga steel sheet in a zinc bath containing a very small amount of Al, theplating layer is substantially composed of the η phase, and the Al-basedoxide layer resulting from Al contained in the zinc bath is formed onthe surface. The η phase is softer than the ξ phase or the δ phase whichis the alloy phase of the hot-dip galvannealed steel sheet, and themelting point of the η phase is lower. Consequently, adhesion is likelyto occur and sliding performance is poor during press forming. However,in the case of the hot-dip galvanized steel sheet, since the Al-basedoxide layer is formed on the surface, an effect of preventing adhesionto the die is slightly exhibited. In particular, when the hot-dipgalvanized steel sheet slides over a die and when the sliding distanceis short, degradation in the sliding performance may not occur. However,since the Al-based oxide layer formed on the surface is thin, as thesliding distance is increased, adhesion becomes likely to occur, and itis not possible to obtain satisfactory press formability under theextended sliding conditions.

In order to prevent adhesion between the hot-dip galvanized steel sheetand the die, it is effective to form a thick oxide layer on the surfaceof the steel sheet. Consequently, it is effective in improving thesliding performance of the hot-dip galvanized steel sheet to form theoxide layer including both the Zn-based oxide and the Al-based oxide bypartially breaking down the Al-based oxide layer on the surface of theplating layer and forming the Zn oxide-based layer by oxidation.

Although the reason for the above is not clear, the sliding performanceis assumed to improve due to the mechanism described below. That is, inthe regions in which the Al-based oxide layer on the plating layer ispartially broken down and a new surface is exposed, the reactivity isincreased, and the Zn-based oxide can be easily generated. In contrast,the region in which the Al-based oxide layer remains is inactive, andthe oxidation does not advance. In the region in which the Zn-basedoxide is formed, since the thickness of the oxide layer can be easilycontrolled, it is possible to obtain the thickness of the oxide layerrequired for improving the sliding performance. During actual pressforming, the die is brought into contact with the oxide layer includingthe Zn-based oxide and the Al-based oxide. Even if the Al-based oxidelayer is scraped away to cause a state in which adhesion easily occurs,since the Zn-based oxide layer can exhibit the adhesion-preventingeffect, it is possible to improve the press formability.

When the thickness of the oxide layer is controlled, if a largethickness is attempted to be obtained, the thickness of the region inwhich the Zn-based oxide is present becomes large and the thickness ofthe region in which the Al-based oxide layer remains does not becomelarge. Consequently, an oxide layer with a nonuniform thickness in whichthick regions and thin regions are present is formed over the entiresurface of the plating layer. However, because of the same mechanism asthat described above, it is possible to improve the sliding performance.In addition, even if the thin regions partially do not include the oxidelayer for some reason, it is possible to improve the sliding performancebecause of the same mechanism.

By setting the average thickness of the oxide layer at 10 nm or more,satisfactory sliding performance can be obtained. To set the averagethickness of the oxide layer at 20 nm or more is more effective. Thereason for this is that in press working in which the contact areabetween the die and the workpiece is large, even if the surface regionof the oxide layer is worn away, the oxide layer remains, and thus thesliding performance is not degraded. On the other hand, although thereis no upper limit for the average thickness of the oxide layer in viewof the sliding performance, if a thick oxide layer is formed, thereactivity of the surface is extremely decreased, and it becomesdifficult to form a chemical conversion coating. Therefore, the averagethickness of the oxide layer is desirably 200 nm or less.

Additionally, the average thickness of the oxide layer can be determinedby Auger electron spectroscopy (AES) combined with Ar ion sputtering. Inthis method, after sputtering is performed to a predetermined depth, thecomposition at the depth is determined based on the correction of thespectral intensities of the individual elements to be measured usingrelative sensitivity factors. The O content resulting from oxidesreaches the maximum value at a certain depth (which may be the outermostlayer), then decreases, and becomes constant. The thickness of the oxideis defined as a depth that corresponds to a half of the sum of themaximum value and the constant value at a position deeper than themaximum value.

It is also possible to check the presence or absence of an oxide layerwith nonuniform thickness based on the measurement results of Augerelectron spectroscopy (AES). This is based on the fact that the thickregions are primarily composed of the Zn-based oxide and the thinregions are composed of the Al-based oxide. The thickness can beevaluated based on the Zn/Al ratio (atomic ratio) at the surface layer.That is, the regions with a Zn/Al ratio exceeding 1.0 correspond tothick regions, and the regions with a Zn/Al ratio of 1.0 or lesscorrespond to thin regions. By performing analysis at given points, andif the Zn/Al ratio at any one point is 1.0 or less, the formation of anoxide layer with a nonuniform thickness can be confirmed. The presenceratio between the thick regions and the thin regions is not particularlylimited. If the area occupied by the thin regions is large, the averagethickness of the oxide layer is less than 10 nm, and the effect ofimproving the sliding performance is not obtained. If the averagethickness is within the range of the present invention, satisfactorycharacteristics can be obtained.

The shape of the region in which the Zn-based oxide is present is notparticularly limited. It has been found that by forming irregularitiesin the surface of the plating layer and by allowing the Zn-based oxideto be present at least on the concavities, the sliding friction can bereduced satisfactorily. The concavities of the surface of the platinglayer, which are different from the concavities of themicroirregularities of the Zn-based oxide region, correspond tomacroirregularities, for example, with such a size that the diameter isabout several to 100 micrometers when the concavity is transposed into acircle with the same area.

The reason for the reduction in the sliding friction is thought to be asfollows. As described above, since the Al-based oxide layer is presenton the surface of the plating layer of the hot-dip galvanized steelsheet, if the sliding distance is short, the sliding friction isrelatively small. As the sliding distance increases, the slidingfriction increases. Under the long sliding conditions, in the case ofthe hot-dip galvanized steel sheet including the plating layersubstantially composed of the η phase which is softer and more easilydeformed compared with the cold rolled steel sheet or the hot-dipgalvannealed steel sheet, not only the convexities but also most of theconcavities of the surface are worn out and the sliding area is greatlyincreased, resulting in an increase in the sliding friction. By formingthe Zn-based oxide which is highly effective in reducing slidingfriction on the concavities of the surface of the plating layer, it ispossible to prevent the sliding area from being increased, resulting ina reduction in the increase of sliding friction under the long slidingconditions.

The thickness distribution of the oxide layer can be directly observedwith a scanning electron microscope using an electron beam at anaccelerating voltage of 1 kV or less (refer to Nonpatent Literature 1:Masayasu Nagoshi and two others, “Actual material surface observed withultra-low voltage scanning electron microscope”, Hyomen Gijutsu (Journalof the Surface Finishing Society of Japan) 2003, 54 (1), 31-34).

In accordance with this method, it is possible to obtain a secondaryelectron image in which the thick regions and the thin regions of theoxide layer can be easily distinguished. The presence ratio of both canbe calculated by processing the image, etc. As a result of evaluation ofthe presence ratio of the thick regions of the oxide applied to thehot-dip galvanized steel sheet using the method, it has been found thatif the thick regions of the oxide have an areal rate of at least 15%with respect to the surface of the plating layer, the sliding frictionis reduced. There is no upper limit for the presence ratio of the thickregions of the oxide regarding the sliding friction reducing effect.

In order to form such an oxide layer, a method is effective in which ahot-dip galvanized steel sheet is brought into contact with an acidicsolution having a pH buffering effect, allowed to stand for 1 to 30seconds, and then washed with water, followed by drying.

Although the mechanism of the formation of the oxide layer is not clear,it is thought to be as follows. When the hot-dip galvanized steel sheetis brought into contact with the acidic solution, zinc on the surface ofthe steel sheet starts to be dissolved. When zinc is dissolved, hydrogenis also generated. Consequently, as the dissolution of zinc advances,the hydrogen ion concentration in the solution decreases, resulting inan increase in the pH of the solution. A Zn-based oxide layer is therebyformed on the surface of the hot-dip galvanized steel sheet. Asdescribed above, in order to form the Zn-based oxide, zinc must bedissolved and the pH of the solution in contact with the steel sheetmust be increased. Therefore, it is effective to adjust the retentiontime after the steel sheet is brought into contact with the acidicsolution until washing with water is performed. If the retention time isless than one second, the liquid is washed away before the pH of thesolution with which the steel sheet is in contact is increased.Consequently, it is not possible to form the oxide. On the other hand,even if the steel sheet is allowed to stand for 30 seconds or more,there is no change in the formation of the oxide.

The acidic solution used for such oxidation preferably has a pH of 1.0to 5.0. If the pH exceeds 5.0, the dissolution rate of zinc isdecreased. If the pH is less than 1.0, the dissolution of zinc isexcessively accelerated. In either case, the formation rate of the oxideis decreased. Preferably, a chemical solution having a pH bufferingeffect is added to the acidic solution. By using such a chemicalsolution, pH stability is imparted to the treatment liquid during theactual production and the increase in the pH required for generating theoxide is also activated, and thereby a thick oxide layer is efficientlyformed.

Any chemical solution which has a pH buffering effect in the acidicrange may be used. Examples thereof include acetates, such as sodiumacetate (CH₃COONa); phthalates, such as potassium hydrogen phthalate((KOOC)₂C₆H₄); citrates, such as sodium citrate (Na₃C₆H₅O₇) andpotassium dihydrogen citrate (KH₂C₆H₅O₇); succinates, such as sodiumsuccinate (Na₂C₄H₄O₄); lactates, such as sodium lactate (NaCH₃CHOHCO₂);tartrates, such as sodium tartrate (Na₂C₄H₄O₆); borates; and phosphates.These may be used alone or in combination of two or more.

The concentration of the chemical solution is preferably 5 to 50 g/l. Ifthe concentration is less than 5 g/l, the pH buffering effect isinsufficient, and it is not possible to form a desired oxide layer. Ifthe concentration exceeds 50 g/l, the effect is saturated, and it alsotakes a long time to form the oxide. By bringing the galvanized steelsheet into contact with the acidic solution, Zn from the plating layeris dissolved in the acidic solution, which does not substantiallyprevent the formation of the Zn oxide. Therefore, the Zn concentrationin the acidic solution is not specifically defined.

The method for bringing the galvanized steel sheet into contact with theacidic solution is not particularly limited. For example, a method inwhich the galvanized steel sheet is immersed in the acidic solution, amethod in which the acidic solution is sprayed to the galvanized steelsheet, or a method in which the acidic solution is applied to thegalvanized steel sheet using an application roller may be employed.Desirably, the acidic solution is applied so as to be present in a thinliquid film form on the surface of the steel sheet. If the amount of theacidic solution present on the surface of the steel sheet is large, evenif zinc is dissolved, the pH of the solution is not increased, and onlythe dissolution of zinc occurs continuously. Consequently, it takes along time to form the oxide layer, and the plating layer is greatlydamaged. The original rust-preventing function of the steel sheet may belost. From this viewpoint, the amount of the liquid film is preferablyadjusted to 3 g/m² or less. The amount of the liquid film can beadjusted by squeeze rolling, air wiping, or the like.

The hot-dip galvanized steel sheet must be temper-rolled before theprocess of forming the oxide layer. The temper rolling operation isusually performed primarily in order to adjust the material quality. Inthe present invention, the temper rolling operation is also performed topartially break down the Al-based oxide layer present on the surface ofthe steel sheet.

The present inventors have observed the surface of the galvanized steelsheet before and after the formation of the oxide using a scanningelectron microscope and found that the Zn-based oxide is mainly formedin the regions in which the Al-based oxide layer is broken down by theconvexities of fine irregularities of the surface of the roller when theroller is brought into contact with the surface of the plating layerduring temper rolling. Consequently, by controlling the roughness of thesurface of the roller for temper rolling and elongation during temperrolling, the area of the broken down Al-based oxide layer can becontrolled, and thereby the areal rate and distribution of the Zn-basedoxide layer can be controlled. Additionally, concavities can also beformed on the surface of the plating layer by such a temper rollingoperation.

The example in which temper rolling is performed has been describedabove. Any other techniques which can mechanically break down theAl-based oxide layer on the surface of the plating layer may beeffective in forming the Zn-based oxide and controlling the areal rate.Examples thereof include processing using a metallic brush and shotblasting.

It is also effective to perform activation treatment after the temperrolling step and before the oxidation step, in which the steel sheet isbrought into contact with an alkaline solution to activate the surface.This treatment is performed to further remove the Al-based oxide and toexpose a new surface. In the temper rolling step described above, theremay be a case in which the Al-based oxide layer is not broken downsufficiently depending on the type of the steel sheet because of theelongation restricted by the material. Therefore, in order to stablyform an oxide layer having excellent sliding performance regardless ofthe type of the steel sheet, it is necessary to activate the surface byfurther removing the Al-based oxide layer.

The method used in order to bring the steel sheet into contact with thealkaline solution is not particularly limited, and immersion or sprayingmay be used. Any alkaline solution enables the activation of thesurface. If the pH is low, the reaction is slow and it takes a long timeto complete the process. Consequently, the alkaline solution preferablyhas a pH of 10 or more. Any type of alkaline solution having the pH inthe above range may be used. For example, sodium hydroxide may be used.

The shape of the Zn-based oxide formed on the surface of the platinglayer has not been described above. By forming microirregularities inthe Zn-based oxide, sliding friction can be further reduced. Themicroirregularities are defined by a surface roughness in which theaverage roughness (Ra) determined based on the roughness curve is 100 nmor less and the mean spacing (S) of local irregularities determinedbased on the roughness curve is 1,000 nm or less.

The sliding friction is reduced by the microirregularities because theconcavities of the microirregularities are believed to function as agroup of fine oil pits so that a lubricant can be effectively caughttherein. That is, in addition to the sliding friction reducing effect asthe oxide, a further sliding friction reducing effect is believed to beexhibited because of the fine sump effect in which the lubricant iseffectively retained in the sliding section. Such a lubricant-retainingeffect of the microirregularities is particularly effective in stablyreducing the sliding friction of the hot-dip galvanized layer which hasa relatively smooth surface macroscopically, in which a lubricant is noteasily retained macroscopically, and on which it is difficult to stablyform a macroscopic surface roughness by rolling or the like in order toachieve lubricity. The lubricant-retaining effect of themicroirregularities is particularly effective under the slidingconditions in which the contact surface pressure is low.

With respect to the structure of the microirregularities, for example,the surface of the Zn-based oxide layer may have microirregularities.Alternatively, a Zn-based oxide in a granular, tabular, or scaly shapemay be distributed directly on the surface of the plating layer or onthe oxide layer and/or hydroxide layer. Desirably, themicroirregularities have Ra of 100 nm or less and S of 800 nm or less.Even if Ra and S are increased from the above upper limits, thelubricant-retaining effect is not substantially improved, and it becomesnecessary to apply the oxide thickly, resulting in a difficulty inproduction. Although the lower limits of the parameters are notparticularly defined, it has been confirmed that the slidingfriction-reducing effect is exhibited at Ra of 3 nm or more and S of 50nm or more. More preferably, Ra is 4 nm or more. If themicroirregularities become too small, the surface becomes close to asmooth surface, resulting in a reduction in the viscous oil-retainingeffect, which is not advantageous.

One of the methods effective in controlling Ra and S is to incorporateFe into the Zn-based oxide as will be described below. If Fe isincorporated into the Zn-based oxide, the Zn oxide gradually becomesfiner and the number of pieces increases. By controlling the Fe contentand the growth time, it is possible to adjust the size and distributionof the Zn oxide, and thereby Ra and S can be adjusted. This is notrestricted by the shape of the microirregularities.

The surface roughness parameters, i.e., Ra and S, can be calculatedaccording to the formulae described in Japan Industrial StandardB-0660-1998 “Surface roughness—Terms”, etc., based on the roughnesscurve with a length of several microns extracted from the digitizedsurface shape of the Zn-based oxide using a scanning electron microscopeor scanning probe microscope (such as an atomic force microscope) havingthree-dimensional shape measuring function. The shape of themicroirregularities can be observed using a high-resolution scanningelectron microscope. Since the thickness of the oxide is small at aboutseveral tens of nanometers, it is effective to observe the surface at alow accelerating voltage, for example, at 1 kV or less. In particular,if the secondary electron image is observed by excluding secondaryelectrons with low energy of about several electron volts as electronenergy, it is possible to reduce contrast caused by the electrostaticcharge of the oxide. Consequently, the shape of the microirregularitiescan be observed satisfactorily (refer to Nonpatent Literature 1).

The method for forming the microirregularities in the Zn-based oxide isnot particularly limited. One of the effective methods is to incorporateFe into the Zn-based oxide. By incorporating Fe into the Zn-based oxide,the size of the Zn-based oxide can be miniaturized. An aggregate of theminiaturized oxide pieces makes microirregularities. Although the reasonwhy the oxide containing Zn and Fe is formed into an oxide havingmicroirregularities is not clear, it is assumed that the growth of theZn oxide is inhibited by Fe or the oxide of Fe. Although the preferableratio (percent) of Fe to the sum of Zn and Fe is not clarified, thepresent inventors have confirmed that the Fe content of at least 1 to 50atomic percent is effective.

Such an oxide containing Zn and Fe is formed by incorporating Fe intothe acidic solution in the method in which the hot-dip galvanized steelsheet is brought into contact with the acidic solution having the pHbuffering effect described above. Although the concentration is notparticularly limited, for example, addition of ferrous sulfate(heptahydrate) in the range of 5 to 400 g/l with the other conditionsbeing the same as those described above enables the formation.

When the hot-dip galvanized steel sheet of the present invention isproduced, Al must be incorporated into the plating bath. The additiveelements other than Al are not particularly limited. That is, theadvantage of the present invention is not degraded even if Pb, Sb, Si,Sn, Mg, Mn, Ni, Ti, Li, Cu, or the like is incorporated besides Al.

The advantage of the present invention is also not degraded even if avery small amount of P, S, N, B, Cl, Na, Mn, Ca, Mg, Ba, Sr, Si, or thelike is incorporated into the oxide layer due to the inclusion ofimpurities during oxidation.

EXAMPLE 1

A hot-dip galvanized layer was formed on a cold-rolled steel sheet witha thickness of 0.8 mm, and then temper rolling was performed. The steelsheet was then immersed in an aqueous sodium acetate solution (20 g/l)with pH of 2.0 at 50° C., allowed to stand for a while, and was washedwith water, followed by drying. Thereby, an oxide layer was formed onthe surface of the plating layer. Twelve samples were thus prepared. Theaverage thickness of the oxide layer was adjusted by changing theretention time. Some of the samples were immersed in an aqueous sodiumhydroxide solution with pH of 12 before the oxidation step.

With respect to each sample, a press formability test was performed andthe thickness of the oxide layer was measured. The press formabilitytest and the measurement of the oxide layer were performed as follows.

(1) Press Formability Test (Coefficient of Friction Measurement Test)

In order to evaluate the press formability, the coefficient of frictionof each sample was measured as follows. FIG. 1 is an elevation viewwhich schematically shows a friction coefficient measuring device. Asshown in the drawing, a test piece 1, which is collected from thesample, for coefficient of friction measurement is fixed on a stage 2,and the stage 2 is fixed on the upper surface of a horizontally movableslide table 3. A vertically movable slide table support 5 including aroller 4 in contact with the lower surface of the slide table 3 isprovided below the slide table 3. A first load cell 7 which measures apressing load N of a bead 6 to the test piece 1 is mounted on the slidetable support 5. A second load cell 8 which measures a sliding frictionF for horizontally moving the slide table 3 with the pressing forcebeing applied is mounted on one end of the slide table 3. Additionally,as a lubricant, cleaning oil for pressing (Preton R352L manufactured bySugimura Chemical Industrial Co., Ltd.) was applied on the surface ofthe test piece 1 when testing was performed.

FIG. 2 is a perspective view which schematically shows the shape anddimension of the bead used. Sliding was performed with the lower surfaceof the bead 6 being pressed against the surface of the test piece 1. Inthe bead 6 shown in FIG. 2, the width is 10 mm, the length in thesliding direction of the test piece is 69 mm, and each edge in thesliding direction of the lower surface of the bead 6 is curved with acurvature of 4.5 mmR. The lower surface of the bead 6 against which thetest piece is pressed has a plane with a width of 10 mm and a length inthe sliding direction of 60 mm. By using this bead, the coefficient offriction under the condition of a long sliding distance can beevaluated. In the coefficient of friction measurement test, the pressingload N was set at 400 kgf and the drawing speed of the test piece (thehorizontal movement speed of the slide table 3) was set at 20 cm/min.

The coefficient of friction between the test piece and the bead wascalculated based,on the equation μ=F/N.

(2) Measurement of Oxide Layer

The contents (atomic percent) of the individual elements were measuredby Auger electron spectoroscopy (AES), and after Ar sputtering wasperformed to a predetermined depth, the contents of the individualelements in the plating layer were measured. By repeating this, thedistribution of each element in the depth direction was measured. The Ocontent resulting from oxides and hydroxides reaches the maximum valueat a certain depth, then decreases, and becomes constant. The thicknessof the oxide was defined as a depth that corresponded to a half of thesum of the maximum value and the constant value at a position deeperthan the maximum value. The average of the thicknesses of the oxidemeasured at 5 given points was defined as the average thickness of theoxide layer. Additionally, as a preliminary treatment, the contaminatedlayer on the surface of each sample was removed by performing Arsputtering for 30 seconds.

When the distributions of the individual elements in the depth directionat given points were measured, it was found that regions in which theZn/Al ratio at the surface layer exceeded 1 and regions in which theZn/Al ratio was 1 or less were mixed. As a result of checking thethicknesses of the oxide layers, it was found that the region with aZn/Al ratio exceeding 1 (region primarily composed of the Zn-basedoxide) had a larger thickness of the oxide layer compared with theregion with a Zn/Al ratio of 1 or less (region primarily composed of theAl-based oxide). Consequently, the average of these regions was definedas the average thickness of the oxide layer.

The test results are shown in Table 1.

TABLE 1 Retention time Sample Alkaline Immersion in until water Averagethickness Coefficient of No. treatment acidic solution washing (sec) ofoxide layer (nm) friction Remarks 1 — — — 6.5 0.280 CE 1 2 — ◯ 0.0 8.80.268 CE 2 3 — ◯ 1.0 11.8 0.230 EP 1 4 — ◯ 5.0 14.5 0.225 EP 2 5 — ◯10.0 18.6 0.218 EP 3 6 — ◯ 20.0 20.3 0.211 EP 4 7 — ◯ 30.0 22.4 0.203 EP5 8 ◯ ◯ 1.0 21.5 0.209 EP 6 9 ◯ ◯ 5.0 25.6 0.198 EP 7 10 ◯ ◯ 10.0 30.10.193 EP 8 11 ◯ ◯ 20.0 32.7 0.189 EP 9 12 ◯ ◯ 30.0 35.5 0.185 EP 10 ◯:Performed CE: Comparative Example EP: Example of Present Invention

The followings are evident from the test results shown in Table 1.

(1) Since Sample No. 1 is not subjected to oxidation treatment aftertemper rolling, the coefficient of friction is high.

(2) Although Sample No. 2 is subjected to oxidation treatment aftertemper rolling, the retention time until water washing is not within therange of the present invention. Consequently, the average thickness ofthe oxide layer on the surface of the plating layer is not within therange of the present invention. The coefficient of friction is lowerthan that of Sample No. 1, but is insufficient.

(3) With respect to each of Sample Nos. 3 to 7, oxidation treatment isperformed after temper rolling and the retention time until waterwashing is within the range of the present invention. Consequently, theaverage thickness of the oxide layer on the surface of the plating layeris within the range of the present invention, and the coefficient offriction is low.

(4) With respect to each of Sample Nos. 8 to 12, immersion in thealkaline solution is performed before oxidation treatment. Thecoefficient of friction is lower compared with each of Sample Nos. 3 to7 with the same retention time until water washing.

EXAMPLE 2

A hot-dip galvanized layer with a Zn coating weight of 60 g/m² wasformed on a cold-rolled steel sheet with a thickness of 0.8 mm, and thentemper rolling was performed with respect to seven samples. Two types oftemper rolling were performed. In temper rolling Type X, rolling wasperforming using a discharge dull roller with a roughness Ra of 3.4 μmso that the elongation was 0.8%. In temper rolling Type Y, rolling wasperformed using a roller with a roughness Ra of 1.4 μm and using a shotblasting technique so that the elongation was 0.7%. Additionally, intemper rolling type Y, with respect to the steel sheet on whichoxidation treatment was not performed, the contact area rate of theroller was evaluated to be about 20% using a scanning electronmicroscope at an accelerating voltage of 0.5 to 2 kV. The contact arearate of the roller was determined by measuring the area of the regionwith which the roller was brought into contact based on a secondaryelectron image of the scanning electron microscope. The surface of theplating layer with which the roller was not brought into contact wasvery smooth, while in the region with which the roller was brought intocontact, the surface was roughened and not smooth. Based on this fact,both can be easily distinguished.

The steel sheet was then immersed in an aqueous sodium acetate solution(40 g/l) with a pH of 1.7 at the working temperature for 3 seconds,allowed to stand for 5 seconds, and was washed with water, followed bydrying. Thereby, an oxide layer was formed on the surface of the platinglayer (treatment liquid A). At this stage, with respect to some of thesamples, the same treatment was performed using, instead of the abovetreatment liquid, an aqueous sodium acetate solution (40 g/l) with pH of2.0 to which ferrous sulfate (heptahydrate) was added. A treatmentliquid B, a treatment liquid C, and a treatment liquid D with a ferroussulfate (heptahydrate) content of 5 g/l, 40 g/l, and 450 g/l,respectively, were used. The temperature of the treatment liquids A, B,and C was 30° C., and the temperature of the treatment liquid D was 20°C. Some of the samples were immersed in an aqueous sodium hydroxidesolution with a pH of 12 before the above treatment.

With respect to each sample, a press formability test, measurement ofthe average thickness of the oxide layer, evaluation of the compositionof the Zn-based oxide layer, measurement of the areal rate of the regionin which the Zn-based oxide was formed, observation of themicroirregularities of the Zn-based oxide, and measurement of thesurface roughness of the Zn-based oxide were performed.

The press formability test and the measurement of the oxide layer wereperformed as in Example 1. When the thickness of the oxide layer wasevaluated using Auger electron spectroscopy, the composition of theZn-based oxide layer was evaluated by qualitative analysis.Additionally, the press formability test in Example 1 was also used toevaluate the coefficient of friction under the sliding conditions of alow contact area pressure.

In order to measure the areal rate of the region in which the Zn-basedoxide was formed, a scanning electron microscope (LEO1530 manufacturedby LEO Company) was used, and a secondary electron image at a lowmagnification was observed at an accelerating voltage of 0.5 kV with anin-lens secondary electron detector. Under these observation conditions,the region in which the Zn-based oxide was formed was clearlydistinguished as dark contrast from the region in which such an oxidewas not formed. The resultant secondary electron image was binarized byan image processing software, and the areal rate of the dark region wascalculated to determine the areal rate of the region in which Zn-basedoxide was formed.

The formation of the microirregularities of the Zn-based oxide wasconfirmed by a method in which, using a scanning electron microscope(LEO1530 manufactured by LEO Company), a secondary electron image at ahigh magnification was observed with an Everhart-Thornly secondaryelectron detector placed in a sample chamber at an accelerating voltageof 0.5 kV.

In order to measure the surface roughness of the Zn-based oxide, a threedimensional electron probe surface roughness analyzer (ERA-8800FEmanufactured by Elionix Inc.) was used. The measurement was performed atan accelerating voltage of 5 kV and a working distance of 15 mm.Sampling distance in the in-plane direction was set at 5 nm or less (atan observation magnification of 40,000 or more). Additionally, in orderto prevent electrostatic charge build-up due to the electron beamirradiation, gold vapor deposition was performed. For each region inwhich the Zn-based oxide was present, 450 or more roughness curves witha length of about 3 μm in the scanning direction of the electron beamwere extracted. At least three locations were measured for each sample.

Based on the roughness curves, using an analysis software attached tothe apparatus, the average surface roughness (Ra) of the roughnesscurves and the mean spacing (S) of local irregularities of the roughnesscurves were calculated. Herein, Ra and S are parameters for evaluatingthe roughness of the microirregularities and the period, respectively.The general definitions of these parameters are described in JapanIndustrial Standard B-0660-1998 “Surface roughness—Terms”, etc. In thepresent invention, the roughness parameters are based on roughnesscurves with a length of several micrometers, and Ra and S are calculatedaccording to the formulae defined in the literature described above.

When the surface of the sample is irradiated with an electron beam,contamination primarily composed of carbon may grow and appear in themeasurement data. Such an influence is likely to become remarkable whenthe region measured is small as in this case. Therefore, when the datawas analyzed, this influence was eliminated using a Spline hyper filterwith a cut-off wavelength corresponding to a half of the length in themeasurement direction (about 3 μm). In order to calibrate the apparatus,SHS Thin Step Height Standard (Steps 18 nm, 88 nm, and 450 nm)manufactured by VLSI standards Inc. traceable to the U.S. nationalresearch institute NIST was used.

The results are shown in Table 2.

TABLE 2 Average Ra (nm) S (nm) Immersion Temper thickness Composition ofof Areal rate (%) Sample Alkaline in acidic rolling of oxide of filmZn-based Zn-based of Zn-based Coefficient No. treatment solution typelayer (nm) applied* oxide oxide oxide of friction Remarks 1 — — X 7.2 —— — — 0.288 CE 1 2 — — Y 5.9 — — — — 0.331 CE 2 A-1 ◯ A X 27.2 Zn—O 92720 95 0.185 EP 1 A-2 29.5 Zn—O 64 560 91 0.188 EP 2 B-1 B 25.3 Zn—Fe—O48 470 89 0.168 EP 3 B-2 24.6 Zn—Fe—O 33 350 85 0.172 EP 4 C-1 — C Y10.8 Zn—Fe—O 5.6 110 19 0.201 EP 5 C-2 11.7 Zn—Fe—O 4.5 80 21 0.207 EP 6D — D Y 12.6 Zn—Fe—O 3.1 100 24 0.229 EP 7 *Main elements detected byAuger electron spectroscopy ◯: Performed CE: Comparative Example EP:Example of Present Invention

(1) In Examples 1 to 7 of the present invention, Auger electronspectroscopy confirms the presence of the Zn-based oxide and theAl-based oxide on the surface of the plating layer. In Examples 1 to 7of the present invention, the coefficient of friction is lower comparedwith Comparative Example 1 or 2 in which oxidation treatment is notperformed, and thereby the sliding friction is reduced. As is evidentfrom this result, excellent press formability is exhibited.

(2) In Examples 1 to 6 of the present invention, microirregularities areclearly observed in the region in which the Zn-based oxide is present bya scanning electron microscope. On the other hand, in Example 7 of thepresent invention, although slight protrusions are present, the surfaceis smoother compared with Examples 1 to 6 of the present invention. InExamples 1 to 6 of the present invention, Ra is 4 μm or more, and inExample 7 of the present invention, Ra is 3.1 nm. Whenmicroirregularities are present in the region in which the Zn-basedoxide is present and Ra is 4 μm or more, the coefficient of friction islower and the sliding friction is further reduced. As is evident fromthis result, excellent press formability is exhibited.

(3) In Examples 3 to 6 of the present invention in whichmicroirregularities are present, the samples are produced using acidicsolutions in which Fe is incorporated, and the oxide layers are composedof oxides containing Zn and Fe. As in these examples, by using an acidicsolution in which Fe is properly incorporated, the size of themicroirregularities can be controlled, and it is possible to form anoxide containing Zn and Fe with microirregularities having an effect ofgreatly reducing sliding friction.

(4) In all of the examples of the present invention, since the arealrate of the region in which the Zn-based oxide is present is 15% ormore, an excellent sliding friction reducing effect is exhibited.

(5) In Examples 5 to 7 of the present invention, most of the Zn-basedoxides are present on the concavities of the plating layers formed bytemper rolling. In these examples, the coefficient of friction is lowercompared with Comparative Example 2 in which the same type of temperrolling is performed, i.e., similar concavities are present on thesurface of the plating layer. As is evident from this result, theZn-based oxide formed on the concavities of the surface of the platinglayer has a sliding friction-reducing effect.

Embodiment 2

The sliding performance of a hot-dip galvanized steel sheet greatlydepends on the surface pressure during sliding because the plating layeris soft unlike a hot-dip galvannealed steel sheet. It has been foundthat the sliding performance is satisfactory if the surface pressure ishigh and that the sliding performance is degraded if the surfacepressure is decreased. Under the conditions of low surface pressure,since the deformation of the surface of the plating layer is small,convexities are mainly brought into contact with a die. It has beenfound that an oxide layer must be formed also on the convexities inorder to further improve the sliding performance of the hot-dipgalvanized steel sheet under the low surface pressure conditions.

The surface of the hot-dip galvanized steel sheet is planar beforetemper rolling is performed. The irregularities of the roller aretransferred to the surface of the plating layer of the hot-dipgalvanized steel sheet by rolling. The concavities of the surface of theplating layer are more active compared with the convexities because theAl-based oxide is mechanically broken down. On the other hand, theconvexities are substantially not deformed by the rolling operation andare generally maintained to be planar. The Al-based oxide on theconvexities of the surface of the plating layer are not substantiallybroken down. Accordingly, the surface of the hot-dip galvanized steelsheet after temper rolling includes active and inactive portionsnonuniformly.

If such a surface is subjected to oxidation treatment, it is possible toform the Zn-based oxide on the concavities. However, the oxide is formedonly on the concavities, and it is difficult to apply the oxide on theplanar portions corresponding to the convexities other than theconcavities.

The present inventors have also found that by formingmicroirregularities in the Zn-based oxide disposed on the surface of theplating layer, sliding performance can be further improved. Themicroirregularities are defined by a surface roughness in which theaverage roughness Ra determined based on the roughness curve is 100 nmor less and the mean spacing S of local irregularities determined basedon the roughness curve is 1,000 nm or less. This surface roughness isone or more orders of magnitude smaller than the surface roughness (Ra:about 1 μm) described in the Patent Literature 1 or 2. Accordingly, thesurface roughness parameters, such as Ra, in the present invention arecalculated based on the roughness curve with a length of severalmicrons, and are different from the general surface roughness parameterswhich define irregularities of the micron (μm) order or more determinedbased on the roughness curve with a length of the millimeter order ormore. In the related literatures, the surface roughness of the hot-dipgalvanized steel sheet is defined, while in the present invention, thesurface roughness of the oxide layer applied to the surface of thehot-dip galvanized steel sheet is defined.

It is not possible to form such microirregularities simply by bringing ahot-dip galvanized steel sheet into contact with an acidic solution,followed by drying. It is possible to form such microirregularities bybringing a hot-dip galvanized steel sheet into contact with an acidicsolution having a pH buffering effect defined in the present invention,and by retaining the steel sheet in this solution for 1 to 30 secondsbefore water washing because of the mechanism which will be describedbelow. The retention time until water washing is important, and theretention time is more preferably 3 to 10 seconds.

If the oxidation treatment is performed after temper rolling, the oxidehaving microirregularities is preferentially formed on the concavitiesof the plating layer formed by the roller. However, it is difficult toform the oxide having microirregularities on the convexities or theplanar portions which are not influenced by the roller. Under thecircumstances, the present inventors have found that it is effective todecrease the amount of the Al-based oxide on the surface to a properamount by performing activation treatment before the oxidationtreatment. Consequently, it is possible to form the oxide havingmicroirregularities which are effective for sliding performance overmost of the surface of the plating layer, and thereby slidingperformance at low surface pressures can be greatly improved.

The Al-based oxide on the surface of the hot-dip galvanized steel sheetaffects chemical conversion treatability and bondability. In thechemical conversion treatment step in the automotive manufacturingprocess, depending on the state of the chemical conversion treatmentsolution, etching performance may be decreased, resulting in noformation of phosphate crystals. In the case of the hot-dip galvanizedsteel sheet, in particular, because of the presence of the inactiveAl-based oxide on the surface, when the etching performance of thechemical conversion treatment solution is insufficient, unevenness islikely to occur. There may be a case in which the Al-based oxide isremoved by alkaline degreasing before chemical conversion treatment andchemical conversion treatment can be performed satisfactorily. Even insuch a case, if alkaline degreasing violates the mild conditions, theeffect is not achieved, resulting in nonuniform distribution of theAl-based oxide. The unevenness after the chemical conversion treatmentleads to unevenness in subsequent electrodeposition and other defects.

In the automotive manufacturing process, adhesives are used for thepurposes of corrosion prevention, vibration isolation, improvement inbonding strength, etc. Some of the adhesives used for cold-rolled steelsheets and Zn—Fe alloy plating are incompatible with the Al-based oxide,and satisfactory bonding strength cannot be achieved.

As described above, chemical conversion treatability and bondability canbe improved by removing the Al-oxide layer on the surface of the hot-dipgalvanized steel sheet. However, since the oxide layer on the surface isremoved, the ability to prevent adhesion to the press die is weakened,resulting in degradation in press formability.

Based on the findings described above, the present invention realizesthe optimum surface state in which sliding performance at low surfacepressures is improved, satisfactory press formability is achieved, andchemical conversion treatability and bondability are also improved, andmoreover, in which all of the above characteristics are exhibited.

Since the hot-dip galvanized steel sheet is usually produced by dippinga steel sheet in a zinc bath containing a very small amount of Al, theplating layer is substantially composed of the η phase, and the Al-basedoxide layer resulting from Al contained in the zinc bath is formed onthe surface. The η phase is softer than the ξ phase or the δ phase whichis the alloy phase of the hot-dip galvannealed steel sheet, and themelting point of the η phase is lower. Consequently, adhesion is likelyto occur and sliding performance is poor during press forming. However,in the case of the hot-dip galvanized steel sheet, since the Al-basedoxide layer is formed on the surface, an effect of preventing adhesionto the die is slightly exhibited. In particular, when the hot-dipgalvanized steel sheet slides over a die and when the sliding distanceis short, degradation in the sliding performance may not occur. However,since the Al-based oxide layer formed on the surface is thin, as thesliding distance is increased, adhesion becomes likely to occur, and itis not possible to obtain satisfactory press formability under theextended sliding conditions. Furthermore, the hot-dip galvanized steelsheet is soft and more easily adheres to the die compared with othertypes of plating. When the surface pressure is low, the slidingperformance is degraded.

In order to prevent adhesion between the hot-dip galvanized steel sheetand the die, it is effective to form a thick oxide layer uniformly onthe surface of the steel sheet. Consequently, it is effective inimproving the sliding performance of the hot-dip galvanized steel sheetto form the oxide layer including both the Zn-based oxide and theAl-based oxide by partially breaking down the Al-based oxide layer onthe surface of the plating layer and forming the Zn oxide-based layer byoxidation. As will be described below, in a more preferred embodiment,Zn-based oxide layer primarily composed of Zn havingmicroirregularities, which is formed according to the method of thepresent invention, covers substantially most of the surface of theplating layer (at an areal rate of 70% or more).

In the regions in which the Al-based oxide layer present on the platinglayer of the galvanized steel sheet is partially broken down by temperrolling or the like and a new surface is exposed, the reactivity isincreased, and the Zn-based oxide can be easily generated. In contrast,the region in which the Al-based oxide layer remains is inactive, andthe oxidation does not advance. In the region in which the Zn-basedoxide is formed, since the thickness of the oxide layer can be easilycontrolled, it is possible to obtain the thickness of the oxide layerrequired for improving the sliding performance. During actual pressforming, the die is brought into contact with the oxide layer includingthe Zn-based oxide and the Al-based oxide. Even if the Al-based oxidelayer is scraped away to cause a state in which adhesion easily occurs,since the Zn-based oxide layer can exhibit the adhesion-preventingeffect, it is possible to improve the press formability.

When the thickness of the oxide layer is controlled, if a largethickness is attempted to be obtained, the thickness of the region inwhich the Zn-based oxide is present becomes large and the thickness ofthe region in which the Al-based oxide layer remains does not becomelarge. Consequently, an oxide layer with a nonuniform thickness in whichthick regions and thin regions are present is formed over the entiresurface of the plating layer. However, because of the same mechanism asthat described above, it is possible to improve the sliding performance.In addition, even if the thin regions partially do not include the oxidelayer for some reason, it is possible to improve the sliding performancebecause of the same mechanism.

By setting the average thickness of the oxide layer at 10 nm or more,satisfactory sliding performance can be obtained. To set the averagethickness of the oxide layer at 20 nm or more is more effective. Thereason for this is that in press working in which the contact areabetween the die and the workpiece is large, even if the surface regionof the oxide layer is worn away, the oxide layer remains, and thus thesliding performance is not degraded. On the other hand, although thereis no upper limit for the average thickness of the oxide layer in viewof the sliding performance, if a thick oxide layer is formed, thereactivity of the surface is extremely decreased, and it becomesdifficult to form a chemical conversion coating. Therefore, the averagethickness of the oxide layer is desirably 200 nm or less.

In the hot-dip galvanized steel sheet, since the Zn-plating layer issofter and has a lower melting point compared with other types ofplating, sliding performance easily changes with the surface pressure,and the sliding performance is low at low surface pressures. In order toovercome this problem, an oxide with a thickness of 10 nm or more (morepreferably 20 nm or more) must also be disposed on the convexitiesand/or planar portions other than the convexities of the surface of theplating layer formed by rolling. Since the concavities are relativelyactive because the Al-based oxide is broken down, the oxide is easilyformed on the concavities. The oxide is not easily formed in otherregions. Consequently, it is effective to decrease the amount of theAl-based oxide by proper activation treatment. The activation treatmentmay be performed by a method in which the Al-oxide is mechanicallyremoved, such as rolling with a roller, shot blasting, or brushing; orby a method in which the Al-oxide is dissolved in an alkaline solution.The activation treatment is important in order to improve the slidingperformance by enlarging the region coated with the oxide and alsoimportant in order to set the Al content in the oxide to a proper valueso that both chemical conversion treatability and bondability areimproved. In the chemical conversion treatment, the reactivity betweenthe Zn of the plating layer and phosphoric acid must be maintained asmuch as possible in the chemical conversion treatment solution. It iseffective to decrease the Al-based oxide component which is hard todissolve in a weakly acidic chemical conversion treatment solution. Inorder to increase the bonding strength with the adhesive, a decrease inthe amount of the Al-based oxide is also effective. An oxide primarilycomposed of Zn with a Zn/Al ratio (atomic concentration ration in theoxide layer) of 4.0 or more is effective. In order to show the effect,the oxide primarily composed of Zn must sufficiently cover the surfaceof the plating layer and must cover a given surface of the plating layerat an areal rate of 70% or more.

The Zn/Al atomic concentration ratio must be 4.0 or more, and this rangealso includes a case in which Al is not present.

The Zn/Al ratio can be measured by Auger electron spectroscopy (AES). Asin the measurement of the oxide layer described above, the distributionof the composition in the depth direction in the planar portion on thesurface of the plating layer is measured. The thickness of the oxidelayer is estimated based on the measurement results, and based on the Znaverage concentration (atomic percent) and the Al average concentration(atomic percent) up to the depth corresponding to the thickness of theoxide layer, the Zn/Al ratio is calculated. However, the composition ofthe oxide formed on the actual surface is not necessarily uniform, andin the very small region of the nm level, portions with a high Alconcentration and portions with a low Al concentration may be present.Consequently, in order to measure the Zn/Al ratio, it is important tomeasure the average composition with respect to a relatively wide regionof about 2 μm×2 μm or more.

In the method in which Auger electron spectoroscopy is performed alongwith sputtering, there is a possibility that the Al concentration may behigher than a value measured based on a cross section obtained by TEM orthe like. Herein, the Zn/Al ratio is defined as the value measured byAuger electron spectroscopy.

The coverage of the oxide primarily composed of Zn with a Zn/Al ratio(atomic concentration ratio in the oxide layer) of 4.0 or more can bemeasured as follows.

In order to display the effect more satisfactorily, the oxide primarilycomposed of Zn with a Zn/Al ratio of 4.0 or more must cover the surfaceof the plating layer sufficiently, and the coverage must be at least 70%on a given surface of the plating layer. The coverage of the oxideprimarily composed of Zn with a Zn/Al ratio of 4.0 or more can bemeasured by element mapping using an X-ray microanalyzer (EPMA) or ascanning electron microscope (SEM). In the EPMA, the intensities or theratio of O, Al, and Zn resulting from the key oxide are preliminarilyobtained, and data of the element mapping measured based on this isprocessed. Thereby, the areal rate can be estimated. On the other hand,it is possible to estimate the areal rate more simply by SEM imageobservation using an electron beam at an accelerating voltage of about0.5 kV. Under this condition, since the portion in which the oxide isformed and the portion in which the oxide is not formed on the surfacecan be clearly distinguished, the areal rate can be measured bybinarizing the resultant secondary electron image using an imageprocessing software. However, it is necessary to preliminarily confirmby AES, EDS, or the like if the observed contrast corresponds to the keyoxide.

By forming microirregularities in the oxide primarily composed of Zn,sliding friction can be further reduced. The microirregularities aredefined by a surface roughness in which the average roughness (Ra)determined based on the roughness curve is about 100 nm or less and themean spacing (S) of local irregularities determined based on theroughness curve is about 1,000 nm or less.

The sliding friction is reduced by the microirregularities because theconcavities of the microirregularities are believed to function as agroup of fine oil pits so that a lubricant can be effectively caughttherein. That is, in addition to the sliding friction reducing effect asthe oxide, a further sliding friction reducing effect is believed to beexhibited because of the fine sump effect in which the lubricant iseffectively retained in the sliding section. Such a lubricant-retainingeffect of the microirregularities is particularly effective in stablyreducing the sliding friction of the hot-dip galvanized layer which hasa relatively smooth surface macroscopically, in which a lubricant is noteasily retained macroscopically, and on which it is difficult to stablyform a macroscopic surface roughness by rolling or the like in order toachieve lubricity. The lubricant-retaining effect of themicroirregularities is particularly effective under the slidingconditions in which the contact surface pressure is low.

With respect to the structure of the microirregularities, for example,the surface of the Zn-based oxide layer may have microirregularities.Alternatively, a Zn-based oxide in a granular, tabular, or scaly shapemay be distributed directly on the surface of the plating layer or onthe oxide layer and/or hydroxide layer. Desirably, themicroirregularities have Ra of 100 nm or less and S of 800 nm or less.Even if Ra and S are increased from the above upper limits, thelubricant-retaining effect is not substantially improved, and it becomesnecessary to apply the oxide thickly, resulting in a difficulty inproduction. Although the lower limits of the parameters are notparticularly defined, it has been confirmed that the slidingfriction-reducing effect is exhibited at Ra of 3 nm or more and S of 50nm or more. More preferably, Ra is 4 nm or more. If themicroirregularities become too small, the surface becomes close to asmooth surface, resulting in a reduction in the viscous oil-retainingeffect, which is not advantageous.

One of the methods effective in controlling Ra and S is to incorporateFe into the Zn-based oxide as will be described below. If Fe isincorporated into the Zn-based oxide, the Zn oxide gradually becomesfiner and the number of pieces increases with the Fe content. Bycontrolling the Fe content and the growth time, it is possible to adjustthe size and distribution of the Zn oxide, and thereby Ra and S can beadjusted. This is not restricted by the shape of themicroirregularities.

The surface roughness parameters, i.e., Ra and S, can be calculatedaccording to the formulae described in Japan Industrial StandardB-0660-1998 “Surface roughness—Terms”, etc., based on the roughnesscurve with a length of several microns extracted from the digitizedsurface shape of the Zn-based oxide using a scanning electron microscopeor scanning probe microscope (such as an atomic force microscope) havingthree-dimensional shape measuring function. The shape of themicroirregularities can be observed using a high-resolution scanningelectron microscope. Since the thickness of the oxide is small at aboutseveral tens of nanometers, it is effective to observe the surface at alow accelerating voltage, for example, at 1 kV or less. In particular,if the secondary electron image is observed by excluding secondaryelectrons with low energy of about several electron volts as electronenergy, it is possible to reduce contrast caused by the electrostaticcharge of the oxide. Consequently, the shape of the microirregularitiescan be observed satisfactorily (refer to Nonpatent Literature 1).

The method for forming the microirregularities in the Zn-based oxide isnot particularly limited. One of the effective methods is to incorporateFe into the Zn-based oxide. By incorporating Fe into the Zn-based oxide,the size of the Zn-based oxide can be miniaturized. An aggregate of theminiaturized oxide pieces makes microirregularities. Although the reasonwhy the oxide containing Zn and Fe is formed into an oxide havingmicroirregularities is not clear, it is assumed that the growth of theZn oxide is inhibited by Fe or the oxide of Fe. Although the preferableratio (percent) of Fe to the sum of Zn and Fe is not clarified, thepresent inventors have confirmed that the Fe content of at least 1 to 50atomic percent is effective. More preferably, the Fe content is 5 to 25atomic percent.

Such an oxide containing Zn and Fe is formed by incorporating Fe into anacidic solution in the method in which the hot-dip galvanized steelsheet is brought into contact with the acidic solution having a pHbuffering effect which will be described below. The preferableconcentration range is 1 to 200 g/l as divalent or trivalent Fe ions.The more preferable concentration range is 1 to 80 g/l. Although themethod for adding Fe ions is not particularly limited, for example, atan Fe ion concentration of 1 to 80 g/l, ferrous sulfate (heptahydrate)may be added in the range of 5 to 400 g/l.

In order to form the oxide layer, a method is effective in which ahot-dip galvanized steel sheet is brought into contact with an acidicsolution having a pH buffering effect, allowed to stand for 1 to 30seconds, and then washed with water, followed by drying.

Although the mechanism of the formation of the oxide layer is not clear,it is thought to be as follows. When the hot-dip galvanized steel sheetis brought into contact with the acidic solution, zinc on the surface ofthe steel sheet starts to be dissolved. When zinc is dissolved, hydrogenis also generated. Consequently, as the dissolution of zinc advances,the hydrogen ion concentration in the solution decreases, resulting inan increase in the pH of the solution. A Zn-based oxide layer is therebyformed on the surface of the hot-dip galvanized steel sheet. Asdescribed above, in order to form the Zn-based oxide, zinc must bedissolved and the pH of the solution in contact with the steel sheetmust be increased. Therefore, it is effective to adjust the retentiontime after the steel sheet is brought into contact with the acidicsolution until washing with water is performed. If the retention time isless than one second, the liquid is washed away before the pH of thesolution with which the steel sheet is in contact is increased.Consequently, it is not possible to form the oxide. On the other hand,even if the steel sheet is allowed to stand for 30 seconds or more,there is no change in the formation of the oxide.

In the present invention, the retention time until washing with water isperformed is important to the formation of the oxide. During theretention period, the oxide (or hydroxide) having the particularmicroirregularities grows. The more preferable retention time is 2 to 10seconds.

The acidic solution used for the oxidation treatment preferably has a pHof 1.0 to 5.0. If the pH exceeds 5.0, the dissolution rate of zinc isdecreased. If the pH is less than 1.0, the dissolution of zinc isexcessively accelerated. In either case, the formation rate of the oxideis decreased. Preferably, a chemical solution having a pH bufferingeffect is added to the acidic solution. By using such a chemicalsolution, pH stability is imparted to the treatment liquid during theactual production. In the process in which the Zn-based oxide is formeddue to the increase in pH in response to the dissolution of Zn, a localincrease in pH is also prevented, and by providing the proper reactiontime, the oxide growth time can be secured. Thereby, the oxide havingmicroirregularities characterized in the present invention iseffectively formed. The anion species of the acidic solution are notparticularly limited, and examples thereof include chloride ions,nitrate ions, and sulfate ions. More preferably, sulfate ions are used.

Any chemical solution which has a pH buffering effect in the acidicrange may be used. Examples thereof include acetates, such as sodiumacetate (CH₃COONa); phthalates, such as potassium hydrogen phthalate((KOOC)₂C₆H₄); citrates, such as sodium citrate (Na₃C₆H₅O₇) andpotassium dihydrogen citrate (KH₂C₆H₅O₇); succinates, such as sodiumsuccinate (Na₂C₄H₄O₄); lactates, such as sodium lactate (NaCH₃CHOHCO₂);tartrates, such as sodium tartrate (Na₂C₄H₄O₆); borates; and phosphates.These may be used alone or in combination of two or more.

The concentration of the chemical solution is preferably 5 to 50 g/l. Ifthe concentration is less than 5 g/l, the pH buffering effect isinsufficient, and it is not possible to form a desired oxide layer. Ifthe concentration exceeds 50 g/l, the effect is saturated, and it alsotakes a long time to form the oxide. By bringing the galvanized steelsheet into contact with the acidic solution, Zn from the plating layeris dissolved in the acidic solution, which does not substantiallyprevent the formation of the Zn-based oxide. Therefore, the Znconcentration in the acidic solution is not specifically defined. As amore preferable pH buffering agent, a solution containing sodium acetatetrihydrate in the range of 10 to 50 g/l, more preferably in the range of20 to 50 g/l, is used. By using such a solution, the oxide of thepresent invention can be effectively obtained.

The method for bringing the galvanized steel sheet into contact with theacidic solution is not particularly limited. For example, a method inwhich the galvanized steel sheet is immersed in the acidic solution, amethod in which the acidic solution is sprayed to the galvanized steelsheet, or a method in which the acidic solution is applied to thegalvanized steel sheet using an application roller may be employed.Desirably, the acidic solution is applied so as to be present in a thinliquid film form on the surface of the steel sheet. If the amount of theacidic solution present on the surface of the steel sheet is large, evenif zinc is dissolved, the pH of the solution is not increased, and onlythe dissolution of zinc occurs continuously. Consequently, it takes along time to form the oxide layer, and the plating layer is greatlydamaged. The original rust-preventing function of the steel sheet may belost. From this viewpoint, the amount of the liquid film is preferablyadjusted to 3 g/m² or less. The amount of the liquid film can beadjusted by squeeze rolling, air wiping, or the like.

The hot-dip galvanized steel sheet must be temper-rolled before theprocess of forming the oxide layer. The temper rolling operation isusually performed primarily in order to adjust the material quality. Inthe present invention, the temper rolling operation is also performed topartially break down the Al-based oxide layer present on the surface ofthe steel sheet.

The present inventors have observed the surface of the galvanized steelsheet before and after the formation of the oxide using a scanningelectron microscope and found that the Zn-based oxide layer is mainlyformed in the regions in which the Al-based oxide layer is broken downby the convexities of fine irregularities of the surface of the rollerwhen the roller is brought into contact with the surface of the platinglayer during temper rolling. Consequently, by controlling the roughnessof the surface of the roller for temper rolling and elongation duringtemper rolling, the area of the broken down Al-based oxide layer can becontrolled, and thereby the areal rate of the region in which theZn-based oxide layer is formed can be controlled. Additionally,concavities can also be formed on the surface of the plating layer bysuch a temper rolling operation.

The example in which temper rolling is performed has been describedabove. Any other techniques which can mechanically break down theAl-based oxide layer on the surface of the plating layer may beeffective in forming the Zn-based oxide and controlling the areal rate.Examples thereof include processing using a metallic brush and shotblasting.

It is also effective to perform activation treatment before theoxidation treatment, in which the steel sheet is brought into contactwith an alkaline solution to activate the surface. This treatment isperformed to further remove the Al-based oxide and to expose a newsurface. In the temper rolling operation described above, there may be acase in which the Al-based oxide layer is not broken down sufficientlydepending on the type of the steel sheet because of the elongationrestricted by the material. Therefore, in order to stably form an oxidelayer having excellent sliding performance regardless of the type of thesteel sheet, it is necessary to activate the surface by further removingthe Al-based oxide layer.

As a result of various research on the Al-based oxide on the surface,which has been obtained when the Al-based oxide layer is removed bycontact with an alkaline solution or the like, before oxidationtreatment, the preferred state of the Al-based oxide layer which iseffective in forming the oxide primarily composed of Zn having themicroirregularities defined in the present invention is as follows.

It is not necessary to completely remove the Al-based oxide on thesurface and the Al-based oxide may be present along with the Zn-basedoxide on the surface of the plating layer. Preferably, the averageconcentration of Al which is contained in the oxide on the planarportions on the surface is less than 20 atomic percent. The Alconcentration is defined as the maximum value of the Al concentrationwithin the depth corresponding to the thickness of the oxide when theaverage thickness of the oxide and the distribution of the Alconcentration in the depth direction in a range of about 2 μm×2 μm aremeasured by Auger electron spectroscopy (AES) and Ar sputtering.

If the Al concentration is 20 atomic percent or more, it becomesdifficult to form the oxide primarily composed of Zn having localmicroirregularities, resulting in a difficulty in covering the surfaceof the plating layer with the oxide primarily composed of Zn at an arealrate of 70% or more. Consequently, sliding performance, in particular,sliding performance under the conditions of low surface pressure,chemical conversion treatability, and bondability are decreased.

In order to produce the state of the Al-based oxide described above,although a mechanical removal method, such as contact with a roller,shot blasting, or brushing may be performed, contact with an aqueousalkaline solution is more effective. In such a case, preferably, the pHof the aqueous solution is set at 11 or more, the bath temperature isset at 50° C. or more, and the contact time with the solution is set tobe one second or more. Any type of solution may be used as long as itspH is within the above range. For example, sodium hydroxide or a sodiumhydroxide-based degreaser may be used.

The activation treatment must be performed before the oxidationtreatment and may be performed before or after the temper rollingoperation performed after hot-dip galvanizing. However, if theactivation treatment is performed after the temper rolling operation,since the Al-based oxide is mechanically broken down at the concavitiesformed by crushing with the roller for temper rolling, the removalamount of the Al oxide tends to vary depending on the concavities andthe convexities and/or planar portions other than the concavities.Consequently, in some case, the amount of the Al oxide may becomenonuniform in the plane after the activation treatment, and thesubsequent oxidation treatment may become nonuniform, resulting in adifficulty obtaining satisfactory characteristics.

Therefore, a process is preferable in which, after plating, activationtreatment is performed first so that a proper amount of the Al oxide isremoved uniformly in the plane, temper rolling is then performed, andsubsequently oxidation treatment is performed.

EXAMPLE 1

A hot-dip galvanized layer was formed on a cold-rolled steel sheet witha thickness of 0.8 mm, and then temper rolling was performed. In somesamples, before or after the temper rolling operation, activationtreatment was performed by bringing the steel sheet into contact with asolution in which the pH was varied by changing the concentration of asodium hydroxide-based degreaser FC-4370 (manufactured by NihonParkerizing Co., Ltd.) for a predetermined time.

Each of the samples subjected to the activation treatment and the temperrolling operation was immersed in a treatment liquid shown in Table 3for 2 to 5 seconds, and the amount of the liquid on the surface of thesample was adjusted to 3 g/m² or less by squeeze rolling. The sample wasleft to stand in air for a predetermined time at room temperature. Thestanding time was changed depending on sample.

TABLE 3 Fe ion Treatment Sodium acetate Ferrous sulfate concentration pHliquid No. trihydrate (g/l) heptahydrate (g/l) (g/l) (Note 1) 1 40 0 0.02 2 40 20 4.0 2 3 40 40 8.0 1.5 4 20 0 0.0 2 5 0 0 0.0 2 6 0 49.8 10.0 2(Note 1) pH was adjusted by sulfuric acid.

With respect to each sample produced as described above, a pressformability test was performed in which sliding performance wasevaluated, and chemical conversion treatability and bondability werealso evaluated. The thickness, distribution, and composition of theoxide layer were also measured. With respect to some of the samples, inorder to confirm the effect of activation treatment, the oxide on thesurface was analyzed before oxidation treatment.

Methods for characteristics evaluation and film analysis will bedescribed below.

(1) Press Formability (Sliding Performance) Evaluation (Measurement ofCoefficient of Friction)

The coefficient of friction of each sample was measured as in the firstembodiment.

(2) Chemical Conversion Treatability

The chemical conversion treatability was evaluated as follows. Arust-preventive oil (NOX-RUST 550HN manufactured by Parker Industries,Inc.) was applied to each sample at about 1 g/m², and then alkalinedegreasing (FC-E2001 manufactured by Nihon Parkerizing Co., Ltd.,spraying, spray pressure: 1 kgf/cm²), water washing, surface preparation(PL-Z manufactured by Nihon Parkerizing Co., Ltd.), and chemicalconversion treatment (PB-L3080 manufactured by Nihon Parkerizing Co.,Ltd.) were performed in that order to form a chemical conversioncoating. The chemical conversion treatment time was set to be constant(2 minutes). In alkaline degreasing, the concentration of the degreasingsolution was set at ½, and the degreasing time was set at 30 seconds,which were milder conditions compared with the standard conditions.

The evaluation was performed based on the appearances after chemicalconversion treatment, using the following criteria.

-   -   ◯: No lack of hiding was observed, and the entire surface was        covered with phosphate crystals.    -   Δ: Lack of hiding was slightly observed.    -   X: The surface included wide regions in which phosphate crystals        were not formed.

(3) Bondability

Oil (Preton R352L manufactured by Sugimura Chemical Industrial Co.,Ltd.) was applied to two test pieces with a dimension of 25×100 mm, anda vinyl chloride resin mastic sealer was applied to a region of 25×10 mmof each test piece. The regions coated with the adhesive were superposedon each other and dried in a drying kiln at 170° C. for 20 minutes toperform bonding. An I-shaped specimen was thereby formed. Tensile forcewas applied to this specimen at 5 mm/min with a tensile tester untilbreak occurred at the bonding position. The maximum load during pullingwas measured. The load was divided by the bonding area to determine abonding strength.

The evaluation criteria were as follows:

-   -   ◯: Bonding strength of 0.2 MPa or more    -   X: Bonding strength of less than 0.2 MPa

(4) Measurement of Thickness of Oxide Layer and Zn/Al Ratio of Oxide

The distribution in the depth direction of composition in the surfaceregion of the plating layer was determined using Auger electronspectroscopy (AES) by repeating Ar⁺ sputtering and AES spectrumanalysis. The sputtering time was converted to the depth according tothe sputtering rate obtained by measuring a SiO₂ film with a knownthickness. The composition (atomic percent) was determined based on thecorrection of the Auger peak intensities of the individual elementsusing relative sensitivity factors. In order to eliminate the influenceof contamination, C was not taken into consideration. The Oconcentration resulting from oxides and hydroxides is high in thevicinity of the surface, decreases with depth, and becomes constant. Thethickness of the oxide is defined as a depth that corresponds to a halfof the sum of the maximum value and the constant value. A region ofabout 2 μm×2 μm in the planar portion was analyzed, and the average ofthe thicknesses measured at 2 to 3 given points was defined as theaverage thickness of the oxide layer. The Zn/Al ratio of the oxide wascalculated based on the Zn average concentration (atomic percent) andthe Al average concentration (atomic percent) in the range correspondingto the thickness of the oxide.

(5) Measurement of Surface State After Activation Treatment

In order to confirm the effect of activation treatment, as in the item(4) described above, the thickness of the oxide and the distribution inthe depth direction of the Al concentration in the planar portion of thesurface after the activation treatment were measured. The maximum Alconcentration in the range corresponding to the thickness of the oxidewas treated as an index of effect of activation treatment.

(6) Measurement of Areal Rate of Oxide Primarily Composed of Zn

In order to measure the areal rate of the oxide primarily composed ofZn, a scanning electron microscope (LEO1530 manufactured by LEO Company)was used, and a secondary electron image at a low magnification wasobserved at an accelerating voltage of 0.5 kV with an in-lens secondaryelectron detector. Under these observation conditions, the region inwhich the oxide primarily composed of Zn was formed was clearlydistinguished as dark contrast from the region in which such an oxidewas not formed. In the strict sense, the brightness distributionobserved may be considered as the thickness distribution of oxides.However, herein, it was confirmed separately by AES that the oxideprimarily composed of Zn with a Zn/Al ratio of 4.0 or more was thickerthan the other oxides, and the dark region was considered as the oxideprimarily composed of Zn with a Zn/Al ratio of 4.0 or more. Theresultant secondary electron image was binarized by an image processingsoftware, and the areal rate of the dark region was calculated todetermine the areal rate of the region in which Zn-based oxide wasformed.

(7) Measurement of Shape of Microirregularities and Roughness Parametersof Oxide

The formation of the microirregularities of the Zn-based oxide wasconfirmed by a method in which, using a scanning electron microscope(LEO1530 manufactured by LEO Company), a secondary electron image at ahigh magnification was observed with an Everhart-Thornly secondaryelectron detector placed in a sample chamber at an accelerating voltageof 0.5 kV.

In order to measure the surface roughness of the Zn-based oxide, a threedimensional electron probe surface roughness analyzer (ERA-8800FEmanufactured by Elionix Inc.) was used. The measurement was performed atan accelerating voltage of 5 kV and a working distance of 15 mm.Sampling distance in the in-plane direction was set at 5 nm or less (atan observation magnification of 40,000 or more). Additionally, in orderto prevent electrostatic charge build-up due to the electron beamirradiation, gold vapor deposition was performed. For each region inwhich the Zn-based oxide was present, 450 or more roughness curves witha length of about 3 μm in the scanning direction of the electron beamwere extracted. At least three locations were measured for each sample.

Based on the roughness curves, using an analysis software attached tothe apparatus, the average surface roughness (Ra) of the roughnesscurves and the mean spacing (S) of local irregularities of the roughnesscurves were calculated. Herein, Ra and S are parameters for evaluatingthe roughness of the microirregularities and the period, respectively.The general definitions of these parameters are described in JapanIndustrial Standard B-0660-1998 “Surface roughness—Terms”, etc. In thepresent invention, the roughness parameters are based on roughnesscurves with a length of several micrometers, and Ra and S are calculatedaccording to the formulae defined in the literature described above.

When the surface of the sample is irradiated with an electron beam,contamination primarily composed of carbon may grow and appear in themeasurement data. Such an influence is likely to become remarkable whenthe region measured is small as in this case. Therefore, when the datawas analyzed, this influence was eliminated using a Spline hyper filterwith a cut-off wavelength corresponding to a half of the length in themeasurement direction (about 3 μm). In order to calibrate the apparatus,SHS Thin Step Height Standard (Steps 18 nm, 88 nm, and 450 nm)manufactured by VLSI standards Inc. traceable to the U.S. nationalresearch institute NIST was used.

The results are shown in Tables 4 and 5.

(1) In Examples of the present invention (Sample Nos. 1 to 7), thesample was subjected to activation treatment using a degreasing liquidin which the concentration was adjusted and the a pH was set at 11 ormore, and then brought into contact with an aqueous solution containingsodium acetate trihydrate as a pH buffering agent as shown in Table 3.By appropriately changing the retention time until washing with water,the oxide layer for each sample was formed. As a result of thesetreatments, the average thickness of oxide layer was 18 to 31 nm, therate of the oxide primarily composed of Zn with a Zn/Al atomicconcentration ratio of 4.0 or more was 90% to 96%. Consequently, thecoefficient of friction was low, and excellent sliding performance wasexhibited. The chemical conversion treatability and bondability werealso satisfactory. In contrast, in each of Comparative Example (SampleNo. 10) in which activation treatment was not performed and ComparativeExample (Sample No. 11) in which the pH for activation treatment wasless than 11, the areal rate of the oxide primarily composed of Zn waslow at 25% or 40%, the coefficient of friction was high, and the slidingperformance was poor. Furthermore, the chemical conversion treatabilityand bondability were inferior to Examples of the present invention.

(2) With respect to each of Sample Nos. 1, 11, and 12, a sample wascollected during activation treatment, the distribution in the depthdirection of the composition in the surface region of the plating layerwas measured using Auger electron spectroscopy (AES) by repeating Ar⁺sputtering and spectrum analysis. The measurement results are shown inFIGS. 3, 4, and 5. As is clear from FIG. 3 showing the Auger profile inthe depth direction of Sample No. 1, the Al concentration of the oxideis less than 20 atomic percent at any depth. In contract, in Sample No.11 (Comparative Example) and Sample No 12 (Comparative Example) shown inFIGS. 4 and 5, the Al concentration is 20 atomic percent or more. Sincethe Sample No. 11 and Sample No. 1 (Example of the present invention)are subjected to oxidation treatment under the same conditions, it isclear that the difference in the areal rate of the oxide primarilycomposed of Zn after oxidation treatment results from the difference inthe Al concentration at the surface obtained by activation treatment.

(3) Among Examples of the present invention, in Sample Nos. 4, 5, and 6,a treatment liquid containing Fe ions was used for oxidation treatment.As a result, 15 to 25 atomic percent of Fe was measured in the oxideprimarily composed of Zn. Although Sample Nos. 3 and 4 are treated undersubstantially the same conditions except for the presence or absence ofFe ions in the treatment liquid, the sliding performance of Sample No. 4containing Fe is slightly more satisfactory than Sample No. 3.

(4) In Sample No. 8 which is Comparative Example, although an acidicsulfuric acid solution is used as the treatment liquid, since a PHbuffering agent is not incorporated therein, the coefficient of frictionis high. The reason for this is believed to be that the areal rate ofthe oxide primarily composed of Zn is low and that the oxide does nothave characteristic microirregularities as provided in the presentinvention. Furthermore, in Sample No. 9, since the oxidation treatmentliquid does not contain a pH buffering agent, satisfactorycharacteristics are not achieved. In Sample Nos. 10 and 11, sinceactivation treatment is not performed sufficiently, the areal rate ofthe oxide primarily composed of Zn is low, and in particular, chemicalconversion treatability and bondability are inferior compared withExamples of the present invention. In Sample No. 12, which is anuntreated hot-dip galvanized steel sheet, the amount of oxide isinsufficient, and sliding performance, chemical conversion treatability,and bondability are inferior compared with Examples of the presentinvention.

TABLE 4 Activation treatment Auger profile of Oxidation treatmentTreatment Before/after surface before Retention time Sample Treatmenttemperature temper rolling oxidation treatment Treatment until water No.liquid pH (° C.) (Note 1) (Note 2) liquid (Table 3) washing (second)Remarks 1 12.5 50 After (FIG. 3) 1 5 EP 2 11 80 After — 1 20 EP 3 12.550 Before — 1 4 EP 4 12.5 60 Before — 2 5 EP 5 12 70 Before — 3 5 EP 612 70 After — 3 5 EP 7 12.5 50 After — 4 5 EP 8 12.5 50 After — 5 5 CE 912.5 50 After — 6 5 CE 10 None — 1 5 CE 11 10.5 50 After (FIG. 4) 1 5 CE12 None (FIG. 5) None CE (Note 1) Timing of activation treatment.Before: before temper rolling After: after temper rolling (Note 2) Augerprofile in the depth direction in the planar portion measured afteractivation treatment and before oxidation treatment EP: Example ofPresent Invention CE: Comparative Example

TABLE 5 Average Areal rate of oxide Fe ratio in oxide thickness ofprimarily composed primarily composed Chemical Sample oxide layer of Zn(Note 3) of Zn (Note 4) Coefficient of conversion No. (nm) (%) (at %)friction treatability Bondability Remarks 1 31 93 — 0.166 ◯ ◯ EP 2 24 92— 0.168 ◯ ◯ EP 3 22 96 — 0.165 ◯ ◯ EP 4 18 91 15 0.155 ◯ ◯ EP 5 18 90 250.158 ◯ ◯ EP 6 22 92 20 0.163 ◯ ◯ EP 7 23 90 — 0.173 ◯ ◯ EP 8 12 45 —0.242 ◯ X CE 9 15 25  5 0.201 ◯ X CE 10 12 25 — 0.193 X X CE 11 16 40 —0.183 Δ X CE 12 8 — — 0.269 X X CE (Note 3) Oxide primarily composed ofZn: Zn/Al atomic concentration ratio of 4.0 or more. Atomicconcentration measuring method and areal rate measuring method aredescribed in the specification. (Note 4) Fe ratio in oxide primarilycomposed of Zn: atomic concentration (at %) defined by Fe/(Zn + Fe).Measurement method is described in the specification. EP: Example ofPresent Invention CE: Comparative Example

Embodiment 3

Since a hot-dip galvanized steel sheet is usually produced by dipping asteel sheet in a zinc bath containing a very small amount of Al, theplating layer is substantially composed of the η phase, and the Al-basedoxide layer resulting from Al contained in the zinc bath is formed onthe surface. The η phase is softer than the ξ phase or the δ phase whichis the alloy phase of a hot-dip galvannealed steel sheet, and themelting point of the η phase is lower. Consequently, adhesion is likelyto occur and sliding performance is poor during press forming. However,in the case of the hot-dip galvanized steel sheet, since the Al-basedoxide layer is formed on the surface, an effect of preventing adhesionto the die is slightly exhibited. In particular, when the hot-dipgalvanized steel sheet slides over a die and when the sliding distanceis short, degradation in the sliding performance may not occur. However,since the Al-based oxide layer formed on the surface is thin, as thesliding distance is increased, adhesion becomes likely to occur, and itis not possible to obtain satisfactory press formability under theextended sliding conditions. Furthermore, the hot-dip galvanized steelsheet is soft and more easily adheres to the die compared with othertypes of plating. When the surface pressure is low, the slidingperformance is degraded.

In order to prevent adhesion between the hot-dip galvanized steel sheetand the die, it is effective to form a thick oxide layer uniformly onthe surface of the steel sheet. Consequently, it is effective inimproving the sliding performance of the hot-dip galvanized steel sheetto form a Zn-based oxide layer by partially breaking down the Al-basedoxide layer on the surface of the plating layer, followed by oxidation.

Furthermore, by incorporating Fe into the Zn-based oxide, a highersliding friction reducing effect can be achieved. Although the reasonfor this is not clear, it is assumed that by forming an oxide containingFe, the adhesion of the oxide is improved, and the sliding frictionreducing effect is likely to be maintained even during sliding. Withrespect to the proper Fe content, it has been confirmed that the Featomic ratio calculated from the expression Fe/(Fe+Zn) based on the Feand Zn atomic concentrations at least in the range of 1% to 50% iseffective. More preferably, by setting the ratio in the range of 5% to25%, the effect can be achieved stably. The Fe and Zn atomicconcentrations in the oxide are most appropriately determined based onthe spectrum measured using a transmission electron microscope (TEM) andan energy dispersive X-ray analyzer (EDS) with respect to a sample ofcross section of the surface layer containing oxide prepared by a FIB-μsampling system. In other methods (e.g., AES and EPMA), it is notpossible to sufficiently decrease the spatial resolution in the regionto be analyzed, and it is difficult to analyze only the oxide on thesurface. Furthermore, it has also been known that incorporation of Feinto the Zn-based oxide to be formed is effective in controlling theamount of the oxide formed and the application and shape (size) ofmicroirregularities which will be described below. Consequently, this isadvantageous in view of stable manufacturing of products.

By setting the average thickness of the Zn-based oxide containing Fe at10 nm or more, satisfactory sliding performance can be obtained. To setthe average thickness of the oxide layer at 20 nm or more is moreeffective. The reason for this is that in press working in which thecontact area between the die and the workpiece is large, even if thesurface region of the oxide layer is worn away, the oxide layer remains,and thus the sliding performance is not degraded. On the other hand,although there is no upper limit for the average thickness of the oxidelayer in view of the sliding performance, if a thick oxide layer isformed, the reactivity of the surface is extremely decreased, and itbecomes difficult to form a chemical conversion coating. Therefore, theaverage thickness of the oxide layer is desirably 200 nm or less.

The average thickness of the oxide layer can be determined by Augerelectron spectroscopy (AES) combined with Ar ion sputtering. In thismethod, after sputtering is performed to a predetermined depth, thecomposition at the depth is determined based on the correction of thespectral intensities of the individual elements to be measured usingrelative sensitivity factors. The O content resulting from oxidesreaches the maximum value at a certain depth (which may be the outermostlayer), then decreases, and becomes constant. The thickness of the oxideis defined as a depth that corresponds to a half of the sum of themaximum value and the constant value at a position deeper than themaximum value. In order to display the effect more satisfactorily, ithas been confirmed that the coverage of the oxide primarily composed ofZn must be at least 15% with respect to a given surface of the platinglayer. The coverage of the oxide primarily composed of Zn can bemeasured by element mapping using an X-ray microanalyzer (EPMA) or ascanning electron microscope (SEM). In the EPMA, the intensities or theratio of O, Al, and Zn resulting from the key oxide are preliminarilyobtained, and data of the element mapping measured based on this isprocessed. Thereby, the areal rate can be estimated. On the other hand,it is possible to estimate the areal rate more simply by SEM imageobservation using an electron beam at an accelerating voltage of about0.5 kV. Under this condition, since the portion in which the oxide isformed and the portion in which the oxide is not formed on the surfacecan be clearly distinguished, the areal rate can be measured bybinarizing the resultant secondary electron image using an imageprocessing software. However, it is necessary to preliminarily confirmby AES, EDS, or the like if the observed contrast corresponds to the keyoxide.

Furthermore, by forming microirregularities in the oxide primarilycomposed of Zn, sliding friction can be further reduced. Themicroirregularities are defined by a surface roughness in which theaverage roughness (Ra) determined based on the roughness curve is about100 nm or less and the mean spacing (S) of local irregularitiesdetermined based on the roughness curve is about 1,000 nm or less. Thesliding friction is reduced by the microirregularities because theconcavities of the microirregularities are believed to function as agroup of fine oil pits so that a lubricant can be effectively caughttherein. That is, in addition to the sliding friction reducing effect asthe oxide, a further sliding friction reducing effect is believed to beexhibited because of the fine sump effect in which the lubricant iseffectively retained in the sliding section. Such a lubricant-retainingeffect of the microirregularities is particularly effective in stablyreducing the sliding friction of the hot-dip galvanized layer which hasa relatively smooth surface macroscopically, in which a lubricant is noteasily retained macroscopically, and on which it is difficult to stablyform a macroscopic surface roughness by rolling or the like in order toachieve lubricity. The lubricant-retaining effect of themicroirregularities is particularly effective under the slidingconditions in which the contact surface pressure is low.

With respect to the structure of the microirregularities, for example,the surface of the Zn-based oxide layer may have microirregularities.Alternatively, a Zn-based oxide in a granular, tabular, or scaly shapemay be distributed directly on the surface of the plating layer or onthe oxide layer and/or hydroxide layer. Desirably, themicroirregularities have Ra of 100 nm or less and S of 1,000 nm or less.Even if Ra and S are increased from the above upper limits, thelubricant-retaining effect is not substantially improved, and it becomesnecessary to apply the oxide thickly, resulting in a difficulty inproduction. Although the lower limits of the parameters are notparticularly defined, it has been confirmed that the slidingfriction-reducing effect is exhibited at Ra of 3 nm or more and S of 50nm or more. More preferably, Ra is 4 nm or more. If themicroirregularities become too small, the surface becomes close to asmooth surface, resulting in a reduction in the viscous oil-retainingeffect, which is not advantageous.

The surface roughness parameters, i.e., Ra and S, can be calculatedaccording to the formulae described in Japan Industrial StandardB-0660-1998 “Surface roughness—Terms”, etc., based on the roughnesscurve with a length of several microns extracted from the digitizedsurface shape of the Zn-based oxide using a scanning electron microscopeor scanning probe microscope (such as an atomic force microscope) havingthree-dimensional shape measuring function. The shape of themicroirregularities can be observed using a high-resolution scanningelectron microscope. Since the thickness of the oxide is small at aboutseveral tens of nanometers, it is effective to observe the surface at alow accelerating voltage, for example, at 1 kV or less. In particular,if the secondary electron image is observed by excluding secondaryelectrons with low energy of about several electron volts as electronenergy, it is possible to reduce contrast caused by the electrostaticcharge of the oxide. Consequently, the shape of the microirregularitiescan be observed satisfactorily (refer to Nonpatent Literature 1).

As described above, by incorporating Fe into the Zn-based oxide, theoxide having microirregularities can be formed, and moreover, it ispossible to control the size of the microirregularities, i.e., Ra and S.By incorporating Fe into the Zn-based oxide, the size of the Zn-basedoxide can be miniaturized. An aggregate of the miniaturized oxide piecesmakes microirregularities. Although the reason why the oxide containingZn and Fe is formed into an oxide having microirregularities is notclear, it is assumed that the growth of the Zn oxide is inhibited by Feor the oxide of Fe.

In order to form the oxide layer, a method is effective in which ahot-dip galvanized steel sheet is brought into contact with an acidicsolution having a pH buffering effect, allowed to stand for 1 to 30seconds, and then washed with water, followed by drying. The Zn-basedoxide containing Fe according to the present invention can be formed byadding Fe into the acidic solution having the pH buffering effect.Although the concentration is not particularly limited, addition offerrous sulfate (heptahydrate) in the range of 5 to 400 g/l enables theformation. However, as described above, in order to set the Fe ratio inthe oxide to be 5% to 25%, more preferably, the ferrous sulfate(heptahydrate) content is in the range of 5 to 200 g/l.

Although the mechanism of the formation of the oxide layer is not clear,it is thought to be as follows. When the hot-dip galvanized steel sheetis brought into contact with the acidic solution, zinc on the surface ofthe steel sheet starts to be dissolved. When zinc is dissolved, hydrogenis also generated. Consequently, as the dissolution of zinc advances,the hydrogen ion concentration in the solution decreases, resulting inan increase in the pH of the solution. A Zn-based oxide layer is therebyformed on the surface of the hot-dip galvanized steel sheet. Asdescribed above, in order to form the Zn-based oxide, zinc must bedissolved and the pH of the solution in contact with the steel sheetmust be increased. Therefore, it is effective to adjust the retentiontime after the steel sheet is brought into contact with the acidicsolution until washing with water is performed. If the retention time isless than one second, the liquid is washed away before the pH of thesolution with which the steel sheet is in contact is increased.Consequently, it is not possible to form the oxide. On the other hand,even if the steel sheet is allowed to stand for 30 seconds or more,there is no change in the formation of the oxide.

In the present invention, the retention time until washing with water isperformed is important to the formation of the oxide. During theretention period, the oxide (or hydroxide) having the particularmicroirregularities grows. The more preferable retention time is 2 to 10seconds.

The acidic solution used for the oxidation treatment preferably has a pHof 1.0 to 5.0. If the pH exceeds 5.0, the dissolution rate of zinc isdecreased. If the pH is less than 1.0, the dissolution of zinc isexcessively accelerated. In either case, the formation rate of the oxideis decreased. Preferably, a chemical solution having a pH bufferingeffect is added to the acidic solution. By using such a chemicalsolution, pH stability is imparted to the treatment liquid during theactual production. In the process in which Zn-based oxide is formed dueto the increase in pH in response to the dissolution of Zn, a localincrease in pH is also prevented, and by providing the proper reactiontime, the oxide growth time can be secured. Thereby, the oxide havingmicroirregularities characterized in the present invention iseffectively formed.

Any chemical solution which has a pH buffering effect in the acidicrange may be used. Examples thereof include acetates, such as sodiumacetate (CH₃COONa); phthalates, such as potassium hydrogen phthalate((KOOC)₂C₆H₄); citrates, such as sodium citrate (Na₃C₆H₅O₇) andpotassium dihydrogen citrate (KH₂C₆H₅O₇); succinates, such as sodiumsuccinate (Na₂C₄H₄O₄) lactates, such as sodium lactate (NaCH₃CHOHCO₂);tartrates, such as sodium tartrate (Na₂C₄H₄O₆); borates; and phosphates.These may be used alone or in combination of two or more.

The concentration of the chemical solution is preferably 5 to 50 g/l. Ifthe concentration is less than 5 g/l, the pH buffering effect isinsufficient, and it is not possible to form a desired oxide layer. Ifthe concentration exceeds 50 g/l, the effect is saturated, and it alsotakes a long time to form the oxide. By bringing the galvanized steelsheet into contact with the acidic solution, Zn from the plating layeris dissolved in the acidic solution, which does not substantiallyprevent the formation of the Zn oxide. Therefore, the Zn concentrationin the acidic solution is not specifically defined. As a more preferablepH buffering agent, a solution containing sodium acetate trihydrate inthe range of 10 to 50 g/l, more preferably in the range of 20 to 50 g/l,is used. By using such a solution, the oxide of the present inventioncan be effectively obtained.

The method for bringing the galvanized steel sheet into contact with theacidic solution is not particularly limited. For example, a method inwhich the galvanized steel sheet is immersed in the acidic solution, amethod in which the acidic solution is sprayed to the galvanized steelsheet, or a method in which the acidic solution is applied to thegalvanized steel sheet using an application roller may be employed.Desirably, the acidic solution is applied so as to be present in a thinliquid film form on the surface of the steel sheet. If the amount of theacidic solution present on the surface of the steel sheet is large, evenif zinc is dissolved, the pH of the solution is not increased, and onlythe dissolution of zinc occurs continuously. Consequently, it takes along time to form the oxide layer, and the plating layer is greatlydamaged. The original rust-preventing function of the steel sheet may belost. From this viewpoint, the amount of the liquid film is preferablyadjusted to 3 g/m² or less. The amount of the liquid film can beadjusted by squeeze rolling, air wiping, or the like.

The hot-dip galvanized steel sheet must be temper-rolled before theprocess of forming the oxide layer. The temper rolling operation isusually performed primarily in order to adjust the material quality. Inthe present invention, the temper rolling operation is also performed topartially break down the Al-based oxide layer present on the surface ofthe steel sheet.

The present inventors have observed the surface of the galvanized steelsheet before and after the formation of the oxide using a scanningelectron microscope and found that the Zn-based oxide is mainly formedin the regions in which the Al-based oxide layer is broken down by theconvexities of fine irregularities of the surface of the roller when theroller is brought into contact with the surface of the plating layerduring temper rolling. Consequently, by controlling the roughness of thesurface of the roller and elongation during temper rolling, the area ofthe broken down Al-based oxide layer can be controlled, and thereby theareal rate and distribution of the Zn-based oxide layer can becontrolled. Additionally, concavities can also be formed on the surfaceof the plating layer by such a temper rolling operation.

The example in which temper rolling is performed has been describedabove. Any other techniques which can mechanically break down theAl-based oxide layer on the surface of the plating layer may beeffective in forming the Zn-based oxide and controlling the areal rate.Examples thereof include processing using a metallic brush and shotblasting.

It is also effective to perform activation treatment before theoxidation treatment, in which the steel sheet is brought into contactwith an alkaline solution to activate the surface. This treatment isperformed to further remove the Al-based oxide and to expose a newsurface. In the temper rolling operation described above, there may be acase in which the Al-based oxide layer is not broken down sufficientlydepending on the type of the steel sheet because of the elongationrestricted by the material. Therefore, in order to stably form an oxidelayer having excellent sliding performance regardless of the type of thesteel sheet, it is necessary to activate the surface by further removingthe Al-based oxide layer.

When the steel sheet is brought into contact with the aqueous alkalinesolution, preferably, the pH of the aqueous solution is set at 11 ormore, the bath temperature is set at 50° C. or more, and the contacttime with the solution is set to be one second or more. Any type ofsolution may be used as long as its pH is within the above range. Forexample, sodium hydroxide or a sodium hydroxide-based degreaser may beused.

The activation treatment must be performed before the oxidationtreatment and may be performed before or after the temper rollingoperation performed after hot-dip galvanizing. However, if theactivation treatment is performed after the temper rolling operation,since the Al-based oxide is mechanically broken down at the concavitiesformed by crushing with the roller for temper rolling, the removalamount of the Al oxide tends to vary depending on the concavities andthe convexities and/or planar portions other than the concavities.Consequently, in some case, the amount of the Al oxide may becomenonuniform in the plane after the activation treatment, and thesubsequent oxidation treatment may become nonuniform, resulting in adifficulty obtaining satisfactory characteristics.

Therefore, a process is preferable in which, after plating, activationtreatment is performed first so that a proper amount of the Al oxide isremoved uniformly in the plane, temper rolling is then performed, andsubsequently oxidation treatment is performed.

When the hot-dip galvanized steel sheet of the present invention isproduced, Al must be incorporated into the plating bath. The additiveelements other than Al are not particularly limited. That is, theadvantage of the present invention is not degraded even if Pb, Sb, Si,Sn, Mg, Mn, Ni, Ti, Li, Cu, or the like is incorporated besides Al. Theadvantage of the present invention is also not degraded even if a verysmall amount of P, S, N, B, Cl, Na, Mn, Ca, Mg, Ba, Sr, Si, or the likeis incorporated into the oxide layer due to the inclusion of impuritiesduring oxidation.

The present invention will be described in more detail based on theexample below.

EXAMPLE

A hot-dip galvanized layer was formed on a cold-rolled steel sheet witha thickness of 0.8 mm, and then temper rolling was performed. Before orafter the temper rolling operation, activation treatment was performedby bringing each sample into contact with a solution of sodiumhydroxide-based degreaser FC-4370 manufactured by Nihon Parkerizing Co.,Ltd. for a predetermined time. In order to form the oxide, each samplesubjected to the activation treatment and the temper rolling operationwas immersed in an acidic solution with varied contents of sodiumacetate trihydrate and ferrous sulfate heptahydrate and with varied pHfor 2 to 5 seconds. The amount of the liquid on the surface of thesample was adjusted to 3 g/m² or less by squeeze rolling, and the samplewas left to stand in air for 5 seconds. For comparison, a sample whichwas not subjected to activation treatment and oxidation treatment (ashot-dip galvanized) and a sample which was subjected to oxidationtreatment without activation treatment were also prepared.

With respect to each sample thus prepared, a press formability test wasperformed in which sliding performance was evaluated, and in order toevaluate the surface shape, the thickness of the oxide layer, thecoverage of the oxide, and the shape of microirregularities weremeasured. Methods for characteristics evaluation and film analysis willbe described below.

(1) Press Formability (Sliding Performance) Evaluation (Measurement ofCoefficient of Friction)

The coefficient of friction of each sample was measured as in the firstembodiment.

(2) Measurement of Fe in Oxide

In order to obtain the Fe ratio in the oxide, a sample of cross sectionof the surface layer containing the oxide prepared by a FIB-μ samplingsystem was measured with a transmission electron microscope (TEM;CM20FEG manufactured by Philips Crop.) and an energy dispersive X-rayanalyzer (EDS; manufactured by EDAX Crop.). The spectrum of the oxidewas measured with EDS, and Fe and Zn atomic concentrations wereestimated based on the peak intensities. The Fe ratio in the oxide wascalculated from the expression Fe/(Fe+Zn).

(3) Measurement of Thickness of Oxide Layer

The distribution in the depth direction of composition on the surface ofthe plating layer was determined using Auger electron spectroscopy (AES)by repeating Ar⁺ sputtering and AES spectrum analysis. The sputteringtime was converted to the depth according to the sputtering rateobtained by measuring a SiO₂ film with a known thickness. Thecomposition (atomic percent) was determined based on the correction ofthe Auger peak intensities of the individual elements using relativesensitivity factors. In order to eliminate the influence ofcontamination, C was not taken into consideration. The O concentrationresulting from oxides and hydroxides is high in the vicinity of thesurface, decreases with depth, and becomes constant. The thickness ofthe oxide is defined as a depth that corresponds to a half of the sum ofthe maximum value and the constant value. A region of about 2 μm×2 μm inthe planar portion was analyzed, and the average of the thicknessesmeasured at 2 to 3 given points was defined as the average thickness ofthe oxide layer.

(4) Measurement of Areal Rate of Oxide Primarily Composed of Zn

In order to measure the areal rate of the oxide primarily composed ofZn, a scanning electron microscope (LEO1530 manufactured by LEO Company)was used, and a secondary electron image at a low magnification wasobserved at an accelerating voltage of 0.5 kV with an in-lens secondaryelectron detector. Under these observation conditions, the region inwhich the oxide primarily composed of Zn was formed was clearlydistinguished as dark contrast from the region in which such an oxidewas not formed. The resultant secondary electron image was binarized byan image processing software, and the areal rate of the dark region wascalculated to determine the areal rate of the region in which Zn-basedoxide was formed.

(5) Measurement of Shape of Microirregularities and Roughness Parametersof Oxide

The formation of the microirregularities of the Zn-based oxide wasconfirmed by a method in which, using a scanning electron microscope(LEO1530 manufactured by LEO Company), a secondary electron image at ahigh magnification was observed with an Everhart-Thornly secondaryelectron detector placed in a sample chamber at an accelerating voltageof 0.5 kV.

In order to measure the surface roughness of the Zn-based oxide, a threedimensional electron probe surface roughness analyzer (ERA-8800FEmanufactured by Elionix Inc.) was used. The measurement was performed atan accelerating voltage of 5 kV and a working distance of 15 mm.Sampling distance in the in-plane direction was set at 5 nm or less (atan observation magnification of 40,000 or more). Additionally, in orderto prevent electrostatic charge build-up due to the electron beamirradiation, gold vapor deposition was performed. For each region inwhich the Zn-based oxide was present, 450 or more roughness curves witha length of about 3 μm in the scanning direction of the electron beamwere extracted. At least three locations were measured for each sample.

Based on the roughness curves, using an analysis software attached tothe apparatus, the average surface roughness (Ra) of the roughnesscurves and the mean spacing (S) of local irregularities of the roughnesscurves were calculated. Herein, Ra and S are parameters for evaluatingthe roughness of the microirregularities and the period, respectively.The general definitions of these parameters are described in JapanIndustrial Standard B-0660-1998 “Surface roughness—Terms”, etc. In thepresent invention, the roughness parameters are based on roughnesscurves with a length of several micrometers, and Ra and S are calculatedaccording to the formulae defined in the literature described above.

When the surface of the sample is irradiated with an electron beam,contamination primarily composed of carbon may grow and appear in themeasurement data. Such an influence is likely to become remarkable whenthe region measured is small as in this case. Therefore, when the datawas analyzed, this influence was eliminated using a Spline hyper filterwith a cut-off wavelength corresponding to a half of the length in themeasurement direction (about 3 μm). In order to calibrate the apparatus,SHS Thin Step Height Standard (Steps 18 nm, 88 nm, and 450 nm)manufactured by VLSI standards Inc. traceable to the U.S. nationalresearch institute NIST was used.

The test results are shown in Table 6. In each of Sample Nos. 1 to 5,the oxide primarily composed of Zn contains a proper amount of Fe andthe coefficient of friction is lower than that of Sample No. 6(Comparative Example) which does not contain Fe.

TABLE 6 Average Areal rate of Fe ratio in thickness of oxide oxideOxidation treatment oxide layer in primarily primarily Sample ActivationFerrous sulfate planar portion composed of Coefficient composed of No.treatment heptahydrate (g/l) pH (nm) Zn (%) of friction Zn (%) Remarks 1Performed 20 2 31 43 0.165  8 EP 2 Performed 40 2 19 82 0.156 18 EP 3Performed 40 2 18 90 0.158 21 EP 4 Performed 40 1.5 22 92 0.163 20 EP 5Performed 80 2 23 95 0.162 25 EP 6 Performed 0 1.5 29 46 0.182  <1* CE 7Not Not performed 5 — 0.281 — CE As performed galvanized *Fe intensitywas less than the lower detection limit of the detector. EP: Example ofPresent Invention CE: Comparative Example

Embodiment 4

Since a hot-dip galvanized steel sheet is usually produced by dipping asteel sheet in a zinc bath containing a very small amount of Al, theplating layer is substantially composed of the η phase, and the Al-basedoxide layer resulting from Al contained in the zinc bath is formed onthe surface. The η phase is softer than the ξ phase or the δ phase whichis the alloy phase of a hot-dip galvannealed steel sheet, and themelting point of the η phase is lower. Consequently, adhesion is likelyto occur and sliding performance is poor during press forming. However,in the case of the hot-dip galvanized steel sheet, since the Al-basedoxide layer is formed on the surface, an effect of preventing adhesionto the die is slightly exhibited. In particular, when the hot-dipgalvanized steel sheet slides over a die and when the sliding distanceis short, degradation in the sliding performance may not occur. However,since the Al-based oxide layer formed on the surface is thin, as thesliding distance is increased, adhesion becomes likely to occur, and itis not possible to obtain satisfactory press formability under theextended sliding conditions. Furthermore, the hot-dip galvanized steelsheet is soft and more easily adheres to the die compared with othertypes of plating. When the surface pressure is low, the slidingperformance is degraded.

In order to prevent adhesion between the hot-dip galvanized steel sheetand the die, it is effective to form a thick oxide layer on the surfaceof the steel sheet. Consequently, it is important to form a Zn-basedoxide layer by partially breaking down the Al-based oxide layer on thesurface of the plating layer, followed by oxidation. Furthermore, byforming the Zn-based oxide so as to have a network structure, slidingfriction can be further decreased. Herein, the network structure isdefined as microirregularities including convexities and discontinuousconcavities surrounded by the convexities. It is not necessary that theconvexities around the concavities have the same height. The heights ofthe convexities may vary to a certain extent. What matters is thatmicroconcavities are dispersed. With respect to the structure of themicroirregularities, for example, the surface of the Zn-based oxidelayer may have microirregularities. Alternatively, a Zn-based oxide in agranular, tabular, or scaly shape may be distributed directly on thesurface of the plating layer or on the oxide layer and/or hydroxidelayer.

The sliding friction is reduced by the microirregularities because theconcavities of the microirregularities are believed to function as agroup of fine oil pits so that a lubricant can be effectively caughttherein. That is, in addition to the sliding friction reducing effect asthe oxide, a further sliding friction reducing effect is believed to beexhibited because of the fine sump effect in which the lubricant iseffectively retained in the sliding section. Such a lubricant-retainingeffect of the microirregularities is particularly effective in stablyreducing the sliding friction of the hot-dip galvanized layer which hasa relatively smooth surface macroscopically, in which a lubricant is noteasily retained macroscopically, and on which it is difficult to stablyform a macroscopic surface roughness by rolling or the like in order toachieve lubricity. The lubricant-retaining effect of themicroirregularities is particularly effective under the slidingconditions in which the contact surface pressure is low.

The size of the microirregularities can be defined by the averageroughness determined based on the roughness curve and the mean spacing Sof local irregularities. In the present invention, it has been confirmedthat the sliding friction reducing effect can be achieved if Ra is inthe range of 4 to 100 nm and S is in the range of 10 to 1,000 nm. Evenif Ra and S are increased from the above upper limits, thelubricant-retaining effect is not substantially improved, and it becomesnecessary to apply the oxide thickly, resulting in a difficulty inproduction. If the microirregularities become too small, the surfacebecomes close to a smooth surface, resulting in a reduction in theviscous oil-retaining effect, which is not advantageous.

In the hot-dip galvanized steel sheet, as will be described below, sincethe concavities to which the roller for temper rolling is brought intocontact with are more active compared with the planar convexities, theoxide is more easily generated. Consequently, in some cases, the oxideformed on the concavities may become coarser than the oxide on theplanar portions. Although such nonuniformity does not degrade theadvantage of the present invention, it has been confirmed that bysetting Ra of the microirregularities of the oxide formed at least onthe planar portions at 500 nm, the sliding friction reducing effect canbe obtained more stably. The reason for this is believed to be thatsince the oxide on the planar portions are directly in contact with thetool during sliding, an adverse effect is produced by the coarse oxidein which the fracture resistance of the oxide is increased rather thanthe lubricant-retaining effect is exhibited.

One of the methods effective in controlling Ra and S is to incorporateFe into the Zn-based oxide as will be described below. If Fe isincorporated into the Zn-based oxide, the Zn oxide gradually becomesfiner and the number of pieces increases with the Fe content. Bycontrolling the Fe content and the growth time, it is possible to adjustthe size and distribution of the Zn oxide, and thereby Ra and S can beadjusted. This is not restricted by the shape of themicroirregularities.

The surface roughness parameters, i.e., Ra and S, can be calculatedaccording to the formulae described in Japan Industrial StandardB-0660-1998 “Surface roughness—Terms”, etc., based on the roughnesscurve with a length of several microns extracted from the digitizedsurface shape of the Zn-based oxide using a scanning electron microscopeor scanning probe microscope (such as an atomic force microscope) havingthree-dimensional shape measuring function. The shape of themicroirregularities can be observed using a high-resolution scanningelectron microscope. Since the thickness of the oxide is small at aboutseveral tens of nanometers, it is effective to observe the surface at alow accelerating voltage, for example, at 1 kV or less. In particular,if the secondary electron image is observed by excluding secondaryelectrons with low energy of about several electron volts as electronenergy, it is possible to reduce contrast caused by the electrostaticcharge of the oxide. Consequently, the shape of the microirregularitiescan be observed satisfactorily (refer to Nonpatent Literature 1).

The method for forming the microirregularities in the Zn-based oxide isnot particularly limited. One of the effective methods is to incorporateFe into the Zn-based oxide. By incorporating Fe into the Zn-based oxide,the size of the Zn-based oxide can be miniaturized. An aggregate of theminiaturized oxide pieces makes microirregularities. Although the reasonwhy the oxide containing Zn and Fe is formed into an oxide havingmicroirregularities is not clear, it is assumed that the growth of theZn oxide is inhibited by Fe or the oxide of Fe. Although the preferableratio (percent) of Fe to the sum of Zn and Fe is not clarified, thepresent inventors have confirmed that the Fe content of at least 1 to 50atomic percent is effective. Such an oxide containing Zn and Fe isformed by incorporating Fe into the acidic solution in the method inwhich the hot-dip galvanized steel sheet is brought into contact withthe acidic solution having the pH buffering effect which will bedescribe below. Although the concentration is not particularly limited,for example, by in incorporating ferrous sulfate (heptahydrate) in therange of 5 to 400 g/l with the other conditions being the same as thosedescribed above, the formation is enabled. In addition, by forming theZn-based oxide having microirregularities so as to cover substantiallymost of the surface of the plating layer (at an areal rate of 70% ormore), the effect of the oxide can be obtained effectively.

In the regions in which the Al-based oxide layer on the plating layer ispartially broken down and a new surface is exposed, the reactivity isincreased, and the Zn-based oxide can be easily generated. In contrast,the region in which the Al-based oxide layer remains is inactive, andthe oxidation does not advance. In the region in which the Zn-basedoxide is formed, since the thickness of the oxide layer can be easilycontrolled, it is possible to obtain the thickness of the oxide layerrequired for improving the sliding performance. During actual pressforming, the die is brought into contact with the oxide layer includingthe Zn-based oxide and the Al-based oxide. Even if the Al-based oxidelayer is scraped away to cause a state in which adhesion easily occursdepending on the sliding conditions, since the Zn-based oxide layer canexhibit the adhesion-preventing effect, it is possible to improve thepress formability.

When the thickness of the oxide layer is controlled, if a largethickness is attempted to be obtained, the thickness of the region inwhich the Zn-based oxide is present becomes large and the thickness ofthe region in which the Al-based oxide layer remains does not becomelarge. Consequently, an oxide layer with a nonuniform thickness in whichthick regions and thin regions are present is formed over the entiresurface of the plating layer. However, because of the same mechanism asthat described above, it is possible to improve the sliding performance.In addition, even if the thin regions partially do not include the oxidelayer for some reason, it is possible to improve the sliding performancebecause of the same mechanism.

By setting the average thickness of the oxide layer at 10 nm or more,satisfactory sliding performance can be obtained. To set the averagethickness of the oxide layer at 20 nm or more is more effective. Thereason for this is that in press working in which the contact areabetween the die and the workpiece is large, even if the surface regionof the oxide layer is worn away, the oxide layer remains, and thus thesliding performance is not degraded. On the other hand, although thereis no upper limit for the average thickness of the oxide layer in viewof the sliding performance, if a thick oxide layer is formed, thereactivity of the surface is extremely decreased, and it becomesdifficult to form a chemical conversion coating. Therefore, the averagethickness of the oxide layer is desirably 200 nm or less.

Additionally, the average thickness of the oxide layer can be determinedby Auger electron spectroscopy (AES) combined with Ar ion sputtering. Inthis method, after sputtering is performed to a predetermined depth, thecomposition at the depth is determined based on the correction of thespectral intensities of the individual elements to be measured usingrelative sensitivity factors. The O content resulting from oxidesreaches the maximum value at a certain depth (which may be the outermostlayer), then decreases, and becomes constant. The thickness of the oxideis defined as a depth that corresponds to a half of the sum of themaximum value and the constant value at a position deeper than themaximum value.

In the hot-dip galvanized steel sheet, since the Zn-plating layer issofter and has a lower melting point compared with other types ofplating, sliding performance easily changes with the surface pressure,and the sliding performance is low at low surface pressures. In order toovercome this problem, an oxide with a thickness of 10 nm or more (morepreferably 20 nm or more) must also be disposed on the convexitiesand/or planar portions other than the convexities of the surface of theplating layer formed by rolling. That is, in order to display the effectmore satisfactorily, the oxide primarily composed of Zn must cover thesurface of the plating layer sufficiently, and the coverage must be atleast 70% on a given surface of the plating layer. The coverage of theoxide primarily composed of Zn can be measured by element mapping usingan X-ray microanalyzer (EPMA) or a scanning electron microscope (SEM).In the EPMA, the intensities or the ratio of O, Al, and Zn resultingfrom the key oxide are preliminarily obtained, and data of the elementmapping measured based on this is processed. Thereby, the areal rate canbe estimated. On the other hand, it is possible to estimate the arealrate more simply by SEM image observation using an electron beam at anaccelerating voltage of about 0.5 kV. Under this condition, since theportion in which the oxide is formed and the portion in which the oxideis not formed on the surface can be clearly distinguished, the arealrate can be measured by binarizing the resultant secondary electronimage using an image processing software. However, it is necessary topreliminarily confirm by AES, EDS, or the like if the observed contrastcorresponds to the key oxide.

In order to form the oxide layer, a method is effective in which ahot-dip galvanized steel sheet is brought into contact with an acidicsolution having a pH buffering effect, allowed to stand for 1 to 30seconds, and then washed with water, followed by drying.

Although the mechanism of the formation of the oxide layer is not clear,it is thought to be as follows. When the hot-dip galvanized steel sheetis brought into contact with the acidic solution, zinc on the surface ofthe steel sheet starts to be dissolved. When zinc is dissolved, hydrogenis also generated. Consequently, as the dissolution of zinc advances,the hydrogen ion concentration in the solution decreases, resulting inan increase in the pH of the solution. A Zn-based oxide layer is therebyformed on the surface of the hot-dip galvanized steel sheet. Asdescribed above, in order to form the Zn-based oxide, zinc must bedissolved and the pH of the solution in contact with the steel sheetmust be increased. Therefore, it is effective to adjust the retentiontime after the steel sheet is brought into contact with the acidicsolution until washing with water is performed. If the retention time isless than one second, the liquid is washed away before the pH of thesolution with which the steel sheet is in contact is increased.Consequently, it is not possible to form the oxide. On the other hand,even if the steel sheet is allowed to stand for 30 seconds or more,there is no change in the formation of the oxide.

In the present invention, the retention time until washing with water isperformed is important to the formation of the oxide. During theretention period, the oxide (or hydroxide) having the particularmicroirregularities grows. The more preferable retention time is 2 to 10seconds.

The acidic solution used for the oxidation treatment preferably has a pHof 1.0 to 5.0. If the pH exceeds 5.0, the dissolution rate of zinc isdecreased. If the pH is less than 1.0, the dissolution of zinc isexcessively accelerated. In either case, the formation rate of the oxideis decreased. Preferably, a chemical solution having a pH bufferingeffect is added to the acidic solution. By using such a chemicalsolution, pH stability is imparted to the treatment liquid during theactual production. In the process in which Zn-based oxide is formed dueto the increase in pH in response to the dissolution of Zn, a localincrease in pH is also prevented, and by providing the proper reactiontime, the oxide growth time can be secured. Thereby, the oxide havingmicroirregularities characterized in the present invention iseffectively formed.

Any chemical solution which has a pH buffering effect in the acidicrange may be used. Examples thereof include acetates, such as sodiumacetate (CH₃COONa); phthalates, such as potassium hydrogen phthalate((KOOC)₂C₆H₄); citrates, such as sodium citrate (Na₃C₆H₅O₇) andpotassium dihydrogen citrate (KH₂C₆H₅O₇); succinates, such as sodiumsuccinate (Na₂C₄H₄O₄); lactates, such as sodium lactate (NaCH₃CHOHCO₂);tartrates, such as sodium tartrate (Na₂C₄H₄O₆); borates; and phosphates.These may be used alone or in combination of two or more.

The concentration of the chemical solution is preferably 5 to 50 g/l. Ifthe concentration is less than 5 g/l, the pH buffering effect isinsufficient, and it is not possible to form a desired oxide layer. Ifthe concentration exceeds 50 g/l, the effect is saturated, and it alsotakes a long time to form the oxide. By bringing the galvanized steelsheet into contact with the acidic solution, Zn from the plating layeris dissolved in the acidic solution, which does not substantiallyprevent the formation of the Zn oxide. Therefore, the Zn concentrationin the acidic solution is not specifically defined. As a more preferablepH buffering agent, a solution containing sodium acetate trihydrate inthe range of 10 to 50 g/l, more preferably in the range of 20 to 50 g/l,is used. By using such a solution, the oxide of the present inventioncan be effectively obtained.

The method for bringing the galvanized steel sheet into contact with theacidic solution is not particularly limited. For example, a method inwhich the galvanized steel sheet is immersed in the acidic solution, amethod in which the acidic solution is sprayed to the galvanized steelsheet, or a method in which the acidic solution is applied to thegalvanized steel sheet using an application roller may be employed.Desirably, the acidic solution is applied so as to be present in a thinliquid film form on the surface of the steel sheet. If the amount of theacidic solution present on the surface of the steel sheet is large, evenif zinc is dissolved, the pH of the solution is not increased, and onlythe dissolution of zinc occurs continuously. Consequently, it takes along time to form the oxide layer, and the plating layer is greatlydamaged. The original rust-preventing function of the steel sheet may belost. From this viewpoint, the amount of the liquid film is preferablyadjusted to 3 g/m² or less. The amount of the liquid film can beadjusted by squeeze rolling, air wiping, or the like.

The hot-dip galvanized steel sheet must be temper-rolled before theprocess of forming the oxide layer. The temper rolling operation isusually performed primarily in order to adjust the material quality. Inthe present invention, the temper rolling operation is also performed topartially break down the Al-based oxide layer present on the surface ofthe steel sheet.

The present inventors have observed the surface of the galvanized steelsheet before and after the formation of the oxide using a scanningelectron microscope and found that the Zn-based oxide layer is mainlyformed in the regions in which the Al-based oxide layer is broken downby the convexities of fine irregularities of the surface of the rollerwhen the roller is brought into contact with the surface of the platinglayer during temper rolling. Consequently, by controlling the roughnessof the surface of the roller for temper rolling and elongation duringtemper rolling, the area of the broken down Al-based oxide layer can becontrolled, and thereby the areal rate and distribution of the Zn-basedoxide layer can be controlled. Additionally, concavities can also beformed on the surface of the plating layer by such a temper rollingoperation.

The example in which temper rolling is performed has been describedabove. Any other techniques which can mechanically break down theAl-based oxide layer on the surface of the plating layer may beeffective in forming the Zn-based oxide and controlling the areal rate.Examples thereof include processing using a metallic brush and shotblasting.

It is also effective to perform activation treatment before theoxidation treatment, in which the steel sheet is brought into contactwith an alkaline solution to activate the surface. This treatment isperformed to further remove the Al-based oxide and to expose a newsurface. In the temper rolling operation described above, there may be acase in which the Al-based oxide layer is not broken down sufficientlydepending on the type of the steel sheet because of the elongationrestricted by the material. Therefore, in order to stably form an oxidelayer having excellent sliding performance regardless of the type of thesteel sheet, it is necessary to activate the surface by further removingthe Al-based oxide layer.

As a result of various research on the Al-based oxide on the surface,which has been obtained when the Al-based oxide layer is removed bycontact with an alkaline solution or the like, the preferred state ofthe Al-based oxide layer which is effective in forming the oxideprimarily composed of Zn having the microirregularities defined in thepresent invention is as follows.

It is not necessary to completely remove the Al-based oxide on thesurface and the Al-based oxide may be present along with the Zn-basedoxide on the surface of the plating layer. Preferably, the averageconcentration of Al which is contained in the oxide on the planarportions on the surface is less than 20 atomic percent. The Alconcentration is defined as the maximum value of the Al concentrationwithin the depth corresponding to the thickness of the oxide when theaverage thickness of the oxide and the distribution of the Alconcentration in the depth direction in a range of about 2 μm×2 μm aremeasured by Auger electron spectroscopy (AES) and Ar sputtering.

If the Al concentration is 20 atomic percent or more, it becomesdifficult to form the oxide primarily composed of Zn having localmicroirregularities, resulting in a difficulty in covering the surfaceof the plating layer with the oxide primarily composed of Zn at an arealrate of 70% or more. Consequently, sliding performance, in particular,sliding performance under the conditions of low surface pressure,chemical conversion treatability, and bondability are decreased.

In order to produce the state of the Al-based oxide described above,contact with an aqueous alkaline solution is effective. In such a case,preferably, the pH of the aqueous solution is set at 11 or more, thebath temperature is set at 50° C. or more, and the contact time with thesolution is set to be one second or more. Any type of solution may beused as long as its pH is within the above range. For example, sodiumhydroxide or a sodium hydroxide-based degreaser may be used.

The activation treatment must be performed before the oxidationtreatment and may be performed before or after the temper rollingoperation performed after hot-dip galvanizing. However, if theactivation treatment is performed after the temper rolling operation,since the Al-based oxide is mechanically broken down at the concavitiesformed by crushing with the roller for temper rolling, the removalamount of the Al oxide tends to vary depending on the concavities andthe convexities and/or planar portions other than the concavities.Consequently, in some case, the amount of the Al oxide may becomenonuniform in the plane after the activation treatment, and thesubsequent oxidation treatment may become nonuniform, resulting in adifficulty obtaining satisfactory characteristics.

Therefore, a process is preferable in which, after plating, activationtreatment is performed first so that a proper amount of the Al oxide isremoved uniformly in the plane, temper rolling is then performed, andsubsequently oxidation treatment is performed.

When the hot-dip galvanized steel sheet of the present invention isproduced, Al must be incorporated into the plating bath. The additiveelements other than Al are not particularly limited. That is, theadvantage of the present invention is not degraded even if Pb, Sb, Si,Sn, Mg, Mn, Ni, Ti, Li, Cu, or the like is incorporated besides Al. Theadvantage of the present invention is also not degraded even if a verysmall amount of P, S, N, B, Cl, Na, Mn, Ca, Mg, Ba, Sr, Si, or the likeis incorporated into the oxide layer due to the inclusion of impuritiesduring oxidation.

The present invention will be described in more detail based on theexample below.

EXAMPLE

A hot-dip galvanized layer was formed on a cold-rolled steel sheet witha thickness of 0.8 mm, and then temper rolling was performed. Before orafter the temper rolling operation, activation treatment was performedby bringing each sample into contact with a solution of sodiumhydroxide-based degreaser FC-4370 manufactured by Nihon Parkerizing Co.,Ltd. for a predetermined time. In order to form the oxide, each samplesubjected to the activation treatment and the temper rolling operationwas immersed in an acidic solution with varied contents of sodiumacetate trihydrate and ferrous sulfate heptahydrate and with varied pHfor 2 to 5 seconds. The amount of the liquid on the surface of thesample was adjusted to 3 g/m² or less by squeeze rolling, and the samplewas left to stand in air for 5 seconds. For comparison, a sample whichwas not subjected to activation treatment and oxidation treatment (ashot-dip galvanized) and a sample which was subjected to oxidationtreatment without activation treatment were also prepared.

With respect to each sample thus prepared, a press formability test wasperformed in which sliding performance was evaluated, and in order toevaluate the surface shape, the thickness of the oxide layer, thecoverage of the oxide, and the shape of microirregularities weremeasured. Methods for characteristics evaluation and film analysis willbe described below.

(1) Press Formability (Sliding Performance) Evaluation (Measurement ofCoefficient of Friction)

The coefficient of friction of each sample was measured as in the firstembodiment.

(2) Measurement of Thickness of Oxide Layer

The distribution in the depth direction of composition on the surface ofthe plating layer was determined using Auger electron spectroscopy (AES)by repeating Ar⁺ sputtering and AES spectrum analysis. The sputteringtime was converted to the depth according to the sputtering rateobtained by measuring a SiO₂ film with a known thickness. Thecomposition (atomic percent) was determined based on the correction ofthe Auger peak intensities of the individual elements using relativesensitivity factors. In order to eliminate the influence ofcontamination, C was not taken into consideration. The O concentrationresulting from oxides and hydroxides is high in the vicinity of thesurface, decreases with depth, and becomes constant. The thickness ofthe oxide is defined as a depth that corresponds to a half of the sum ofthe maximum value and the constant value. A region of about 2 μm×2 μm inthe planar portion was analyzed, and the average of the thicknessesmeasured at 2 to 3 given points was defined as the average thickness ofthe oxide layer.

(3) Measurement of Areal Rate of Oxide Primarily Composed of Zn

In order to measure the areal rate of the oxide primarily composed ofZn, a scanning electron microscope (LEO1530 manufactured by LEO Company)was used, and a secondary electron image at a low magnification wasobserved at an accelerating voltage of 0.5 kV with an in-lens secondaryelectron detector. Under these observation conditions, the region inwhich the oxide primarily composed of Zn was formed was clearlydistinguished as dark contrast from the region in which such an oxidewas not formed. The resultant secondary electron image was binarized byan image processing software, and the areal rate of the dark region wascalculated to determine the areal rate of the region in which Zn-basedoxide was formed.

(4) Measurement of Shape of Microirregularities and Roughness Parametersof Oxide

The formation of the microirregularities of the Zn-based oxide wasconfirmed by a method in which, using a scanning electron microscope(LEO1530 manufactured by LEO Company), a secondary electron image at ahigh magnification was observed with an Everhart-Thornly secondaryelectron detector placed in a sample chamber at an accelerating voltageof 0.5 kV.

In order to measure the surface roughness of the Zn-based oxide, a threedimensional electron probe surface roughness analyzer (ERA-8800FEmanufactured by Elionix Inc.) was used. The measurement was performed atan accelerating voltage of 5 kV and a working distance of 15 mm.Sampling distance in the in-plane direction was set at 5 nm or less (atan observation magnification of 40,000 or more). Additionally, in orderto prevent electrostatic charge build-up due to the electron beamirradiation, gold vapor deposition was performed. For each region inwhich the Zn-based oxide was present, 450 or more roughness curves witha length of about 3 μm in the scanning direction of the electron beamwere extracted. At least three locations were measured for each sample.

Based on the roughness curves, using an analysis software attached tothe apparatus, the average surface roughness (Ra) of the roughnesscurves and the mean spacing (S) of local irregularities of the roughnesscurves were calculated. Herein, Ra and S are parameters for evaluatingthe roughness of the microirregularities and the period, respectively.The general definitions of these parameters are described in JapanIndustrial Standard B-0660-1998 “Surface roughness—Terms”, etc. In thepresent invention, the roughness parameters are based on roughnesscurves with a length of several micrometers, and Ra and S are calculatedaccording to the formulae defined in the literature described above.

When the surface of the sample is irradiated with an electron beam,contamination primarily composed of carbon may grow and appear in themeasurement data. Such an influence is likely to become remarkable whenthe region measured is small as in this case. Therefore, when the datawas analyzed, this influence was eliminated using a Spline hyper filterwith a cut-off wavelength corresponding to a half of the length in themeasurement direction (about 3 μm). In order to calibrate the apparatus,SHS Thin Step Height Standard (Steps 18 nm, 88 nm, and 450 nm)manufactured by VLSI standards Inc. traceable to the U.S. nationalresearch institute NIST was used.

The test results are shown in Table 6. The followings are evident fromthe results shown in Table 6.

In each of Sample Nos. 1 to 6, since the thickness of the oxideprimarily composed of Zn formed in the planar portion, the areal rate,and the shape of microirregularities are in the ranges of the presentinvention, the coefficient of friction are low.

In Sample No. 7, the thickness of the oxide primarily composed of Zn andthe areal rate are satisfactory. However, since microirregularities arenot formed properly, the reduction in the coefficient of friction issmall.

In Sample No. 8, since activation treatment is not performed, the oxideis not formed sufficiently.

TABLE 7 Average Oxidation treatment thickness of Areal rate of Shape ofmicroirregularities of oxide Sodium Ferrous oxide layer oxide primarilycomposed of Zn acetate sulfate in planar primarily Temper-rolled SampleActivation trihydrate heptahydrate portion composed of CoefficientPlanar portion concavity No. treatment (g/l) (g/l) pH (nm) Zn (%) offriction Ra (nm) S (nm) Ra (nm) S (nm) Remarks 1 Performed 40 0 1.5 2891 0.176 71 540 82   780 EP 2 Performed 40 0 2 24 93 0.167 45 421 47 433EP 3 Performed 40 0 2 18 91 0.160 11 168 52 612 EP 4 Performed 40 40 221 96 0.156 13 124 13 131 EP 5 Performed 40 80 2 23 95 0.162  5.2  42 4.6  46 EP 6 Performed 40 0 3 17 98 0.169  4.2 113 49 523 EP 7Performed 20 0 4 13 92 0.182  2.3  53 23 421 CE 8 Not performed 40 0 2 812 0.250 — — 18 620 CE 9 Not performed Not performed 5 — 0.281  1.3* 64*  1.6*  70* CE *Original irregularities of the surface of theplating layer instead of the oxide primarily composed of Zn EP: Exampleof Present Invention CE: Comparative Example

1. A hot-dip galvanized steel sheet comprising: a plating layerconsisting essentially of a η phase; and an oxide layer disposed on asurface of the plating layer, said oxide layer having an averagethickness of 10 nm or more; and the oxide layer comprising a Zn-basedoxide layer and an Al-based oxide layer, the Zn-based oxide layer havinga Zn/Al atomic concentration ratio of more than 1 and the Al-based oxidelayer having a Zn/Al atomic concentration ratio of less than 1, whereinthe Zn-based oxide layer has microirregularities; and themicroirregularities have a mean spacing (S) determined based on aroughness curve of 1,000 nm or less and an average roughness (Ra) of 100nm or less.
 2. The hot-dip galvanized steel sheet according to claim 1,wherein the plating layer has concavities and convexities on the surfacethereof; and the Zn-based oxide layer is disposed at least on theconcavities.
 3. The hot-dip galvanized steel sheet according to claim 1,wherein the Zn-based oxide layer comprises an oxide containing Zn andFe; and the Zn-based oxide layer has a Fe atomic concentration ratio of1 to 50 atomic percent, the atomic concentration ratio being defined byan expression Fe/(Zn+Fe).
 4. The hot-dip galvanized steel sheetaccording to claim 1, wherein the Zn-based oxide layer has an areal rateof 15% or more with respect to the surface of the plating layer.
 5. Thehot-dip galvanized steel sheet according to claim 1, wherein the oxidelayer has an average thickness of 10 to 200 nm.
 6. The hot-dipgalvanized steel sheet according to claim 1, wherein the Zn-based oxidelayer has microirregularities with a network structure includingconvexities and discontinuous concavities surrounded by the convexities.7. The hot-dip galvanized steel sheet according to claim 1, wherein theZn-based oxide layer has a Zn/Al atomic concentration ratio of 4 ormore.
 8. The hot-dip galvanized steel sheet according to claim 7,wherein the Zn-based oxide layer has an areal rate of 70% or more withrespect to the surface of the plating layer.
 9. The hot-dip galvanizedsteel sheet according to claim 7, wherein the Zn-based oxide layer isdisposed on the concavities of the surface of the plating layer formedby temper rolling, and on the convexities or planar portions other thanthe concavities.
 10. The hot-dip galvanized steel sheet according toclaim 7, wherein the Zn-based oxide layer comprises an oxide containingZn and Fe; and the Zn-based oxide layer has a Fe atomic concentrationratio defined by an expression Fe/(Zn+Fe) being 1 to 50 atomic percent.11. The hot-dip galvanized steel sheet according to claim 7, wherein theZn-based oxide layer has microirregularities; and the Zn-based oxidelayer has a network structure that is formed by convexities anddiscontinuous concavities surrounded by the convexities.
 12. A hot-dipgalvanized steel sheet, comprising a plating layer consistingessentially of a η phase; and a Zn-based oxide layer containing Fedisposed on the surface of the plating layer, the Zn-based oxide layerhaving an Fe atomic concentration ratio of 1 to 50 atomic percent, theFe atomic concentration ratio being defined by the expressionFe/(Fe+Zn), wherein the Zn-based oxide layer has a mean spacing (S)determined based on a roughness curve being 10 to 1,000 nm and anaverage roughness (Ra) of 4 to 100 nm.
 13. The hot-dip galvanized steelsheet according to claim 12, wherein the Zn-based oxide layer hasmicroirregularities with a network structure including convexities anddiscontinuous concavities surrounded by the convexities.
 14. The hot-dipgalvanized steel sheet according to claim 12, wherein the Zn-based oxidelayer has an areal rate of 15% or more with respect to the surface ofthe plating layer.
 15. A hot-dip galvanized steel sheet, comprising aplating layer consisting essentially of a η phase; and a Zn-based oxidelayer containing Fe disposed on a surface of the plating layer, theZn-based oxide layer having microirregularities with a network structureincluding convexities and discontinuous concavities surrounded by theconvexities. wherein the Zn-based oxide layer has a mean spacing (S)determined based on a roughness curve being 10 to 1,000 nm and anaverage roughness (Ra) of 4 to 100 nm.
 16. The hot-dip galvanized steelsheet according to claim 15, wherein the Zn-based oxide layer has anareal rate of 70% or more with respect to the surface of the platinglayer.
 17. The hot-dip galvanized steel sheet according to claim 15,wherein the Zn-based oxide layer is disposed on the planar portions ofthe surface of the plating layer other than the concavities formed bytemper rolling.
 18. The hot-dip galvanized steel sheet according toclaim 17, wherein, the Zn-based oxide layer, which is disposed on theplanar portions, has a mean spacing (S) determined based on theroughness curve of 10 to 500 nm and the average roughness (Ra) of 4 to100 nm.